Add OpenAPI annotations to v2 snapshot and backup APIs

SOLR-16346 introduced a build integration to produce an OpenAPI spec, but it did little to decorate individual APIs with metadata and documentation that would be picked up by that approach. Individual APIs themselves still need annotated. (See SOLR-16380 for a discussion of the benefits of doing this.)

This ticket targets Solr's snapshot and backup APIs (both at the core and collection level).