API: Add scope to metadata configuration API (#14530)

Resolves #14125.

This PR adds `scope` to the metadata configuration API endpoint,
`/1.0/metadata/configuration`.

Summary of changes:
- Adds api extension, `metadata_configuration_scope`;
- Adds `Scope` field to `MetadataConfigurationConfigKey` struct;
- Adds `scope` to metadata for member specific config keys in
`lxd/network/driver_{bridge/macvlan/physical/sriov}.go`;
- Updates API docs and metadata.

doc/api-extensions.md

@@ -2523,3 +2523,7 @@ Adds support for using the Container Device Interface (CDI) specification to con
 ## `images_all_projects`
 
 This adds support for listing images across all projects using the `all-projects` parameter in `GET /1.0/images` requests.
+
+## `metadata_configuration_scope`
+
+This adds scope metadata to `GET /1.0/metadata/configuration`. Options marked with a `global` scope are applied to all cluster members. Options with a `local` scope must be set on a per-member basis.