feat: provide a way to list installed system extensions

```bash
$ talosctl -n 172.20.0.2 get extensions
NODE         NAMESPACE   TYPE              ID                                       VERSION   NAME     VERSION
172.20.0.2   runtime     ExtensionStatus   000.ghcr.io-smira-gvisor-c927b54-dirty   1         gvisor   20220117.0-v1.0.0
```

```bash
$ talosctl -n 172.20.0.2 get extensions -o yaml
node: 172.20.0.2
metadata:
    namespace: runtime
    type: ExtensionStatuses.runtime.talos.dev
    id: 000.ghcr.io-smira-gvisor-c927b54-dirty
    version: 1
    owner: runtime.ExtensionStatusController
    phase: running
    created: 2022-01-26T20:56:51Z
    updated: 2022-01-26T20:56:51Z
spec:
    image: 000.ghcr.io-smira-gvisor-c927b54-dirty.sqsh
    metadata:
        name: gvisor
        version: 20220117.0-v1.0.0
        author: Andrew Rynhard
        description: |
            This system extension provides gVisor using containerd's runtime handler.
        compatibility:
            talos:
                version: '> v0.15.0-alpha.1'
```

Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>

internal/app/machined/pkg/controllers/runtime/extension_status.go

@@ -0,0 +1,79 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package runtime
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+
+	"github.com/cosi-project/runtime/pkg/controller"
+	"github.com/cosi-project/runtime/pkg/resource"
+	"go.uber.org/zap"
+
+	"github.com/talos-systems/talos/pkg/machinery/constants"
+	"github.com/talos-systems/talos/pkg/machinery/extensions"
+	"github.com/talos-systems/talos/pkg/machinery/resources/runtime"
+)
+
+// ExtensionStatusController loads extensions.yaml and updates ExtensionStatus resources.
+type ExtensionStatusController struct{}
+
+// Name implements controller.Controller interface.
+func (ctrl *ExtensionStatusController) Name() string {
+	return "runtime.ExtensionStatusController"
+}
+
+// Inputs implements controller.Controller interface.
+func (ctrl *ExtensionStatusController) Inputs() []controller.Input {
+	return nil
+}
+
+// Outputs implements controller.Controller interface.
+func (ctrl *ExtensionStatusController) Outputs() []controller.Output {
+	return []controller.Output{
+		{
+			Type: runtime.ExtensionStatusType,
+			Kind: controller.OutputExclusive,
+		},
+	}
+}
+
+// Run implements controller.Controller interface.
+func (ctrl *ExtensionStatusController) Run(ctx context.Context, r controller.Runtime, logger *zap.Logger) error {
+	// controller runs once, as extensions are static
+	select {
+	case <-ctx.Done():
+		return nil
+	case <-r.EventCh():
+	}
+
+	var cfg extensions.Config
+
+	if err := cfg.Read(constants.ExtensionsRuntimeConfigFile); err != nil {
+		if errors.Is(err, io.EOF) {
+			// no extensions installed
+			return nil
+		}
+
+		return fmt.Errorf("failed loading extensions config: %w", err)
+	}
+
+	for _, layer := range cfg.Layers {
+		id := strings.TrimSuffix(layer.Image, ".sqsh")
+
+		if err := r.Modify(ctx, runtime.NewExtensionStatus(runtime.NamespaceName, id), func(res resource.Resource) error {
+			*res.(*runtime.ExtensionStatus).TypedSpec() = *layer
+
+			return nil
+		}); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}

internal/app/machined/pkg/runtime/v1alpha2/v1alpha2_controller.go

@@ -187,6 +187,7 @@ func (ctrl *Controller) Run(ctx context.Context, drainer *runtime.Drainer) error
 			Cmdline:        procfs.ProcCmdline(),
 			Drainer:        drainer,
 		},
+		&runtimecontrollers.ExtensionStatusController{},
 		&runtimecontrollers.KernelModuleConfigController{},
 		&runtimecontrollers.KernelModuleSpecController{
 			V1Alpha1Mode: ctrl.v1alpha1Runtime.State().Platform().Mode(),

internal/app/machined/pkg/runtime/v1alpha2/v1alpha2_state.go

@@ -124,6 +124,7 @@ func NewState() (*State, error) {
 		&network.TimeServerSpec{},
 		&perf.CPU{},
 		&perf.Memory{},
+		&runtime.ExtensionStatus{},
 		&runtime.KernelModuleSpec{},
 		&runtime.KernelParamSpec{},
 		&runtime.KernelParamDefaultSpec{},

pkg/machinery/resources/runtime/extension_status.go

@@ -0,0 +1,84 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package runtime
+
+import (
+	"fmt"
+
+	"github.com/cosi-project/runtime/pkg/resource"
+	"github.com/cosi-project/runtime/pkg/resource/meta"
+
+	"github.com/talos-systems/talos/pkg/machinery/extensions"
+)
+
+// ExtensionStatusType is type of Extension resource.
+const ExtensionStatusType = resource.Type("ExtensionStatuses.runtime.talos.dev")
+
+// ExtensionStatus resource holds status of installed system extensions.
+type ExtensionStatus struct {
+	md   resource.Metadata
+	spec ExtensionStatusSpec
+}
+
+// ExtensionStatusSpec is the spec for system extensions.
+type ExtensionStatusSpec = extensions.Layer
+
+// NewExtensionStatus initializes a ExtensionStatus resource.
+func NewExtensionStatus(namespace resource.Namespace, id resource.ID) *ExtensionStatus {
+	r := &ExtensionStatus{
+		md:   resource.NewMetadata(namespace, ExtensionStatusType, id, resource.VersionUndefined),
+		spec: ExtensionStatusSpec{},
+	}
+
+	r.md.BumpVersion()
+
+	return r
+}
+
+// Metadata implements resource.Resource.
+func (r *ExtensionStatus) Metadata() *resource.Metadata {
+	return &r.md
+}
+
+// Spec implements resource.Resource.
+func (r *ExtensionStatus) Spec() interface{} {
+	return r.spec
+}
+
+func (r *ExtensionStatus) String() string {
+	return fmt.Sprintf("runtime.ExtensionStatus.(%q)", r.md.ID())
+}
+
+// DeepCopy implements resource.Resource.
+func (r *ExtensionStatus) DeepCopy() resource.Resource {
+	return &ExtensionStatus{
+		md:   r.md,
+		spec: r.spec,
+	}
+}
+
+// ResourceDefinition implements meta.ResourceDefinitionProvider interface.
+func (r *ExtensionStatus) ResourceDefinition() meta.ResourceDefinitionSpec {
+	return meta.ResourceDefinitionSpec{
+		Type:             ExtensionStatusType,
+		Aliases:          []resource.Type{"extensions"},
+		DefaultNamespace: NamespaceName,
+		PrintColumns: []meta.PrintColumn{
+			{
+				Name:     "Name",
+				JSONPath: `{.metadata.name}`,
+			},
+			{
+				Name:     "Version",
+				JSONPath: `{.metadata.version}`,
+			},
+		},
+	}
+}
+
+// TypedSpec allows to access the ExtensionStatusSpec with the proper type.
+func (r *ExtensionStatus) TypedSpec() *ExtensionStatusSpec {
+	return &r.spec
+}

pkg/machinery/resources/runtime/runtime_test.go

@@ -25,6 +25,7 @@ func TestRegisterResource(t *testing.T) {
 	resourceRegistry := registry.NewResourceRegistry(resources)
 
 	for _, resource := range []resource.Resource{
+		&runtime.ExtensionStatus{},
 		&runtime.KernelModuleSpec{},
 		&runtime.KernelParamSpec{},
 		&runtime.KernelParamStatus{},