Add @ClusterInternalAPI annotation

For internal cluster REST and JSON communication APIs, which cannot be broken and removed completely in a single Druid release, but can be first superseded by an alternative, and then removed in a subsequent Druid release.

Related: [Add @MetricsAPI annotation](https://github.com/apache/incubator-druid/issues/7478)

Start by reviewing the related “Add @MetricsAPI annotation” issue (#7478) and the Druid conventions it establishes for API annotations. Identify the internal cluster REST and JSON communication APIs covered by this request, then verify that the annotation clearly supports superseding an API before later removal.

Written by the indexing model from the issue text.