s3:idmap: add low_id and high_id to the idmap_domain struct

This global data will replace the backend-specific filter_low_id
and filter_high_id. The presence of a range is generic to all
idmap configs.

source3/include/idmap.h

@@ -33,6 +33,8 @@
 struct idmap_domain {
 	const char *name;
 	struct idmap_methods *methods;
+	uint32_t low_id;
+	uint32_t high_id;
 	void *private_data;
 };