Our prototype Cassandra backend for the main archive currently lacks the metadata_provider_add, metadata_provider_get, metadata_provider_get_by endpoints.

We should add them to swh/storage/cassandra/storage.py, and add the related table in swh/storage/cassandra/schema.py, to make them behave like in the other backends (see their documentation in swh/storage/interface.py); and unskip tests in swh/storage/tests/test_cassandra.py.

(Note: this task is tagged "easy hack" as it doesn't require extensive knowledge of the Software Heritage codebase, but it will require some non-negligible work)