This PR allows the ability to generate `secrets.yaml` (`talosctl gen secrets`) using a Kubernetes PKI directory path (e.g. `/etc/kubernetes/pki`) as input. Also introduces the flag `--kubernetes-bootstrap-token` to be able to set a static Kubernetes bootstrap token to the generated `secrets.yaml` file instead of a randomly-generated one. Closes siderolabs/talos#5894. Signed-off-by: Utku Ozdemir <utku.ozdemir@siderolabs.com>