Implement origin_metadata endpoints in swh/storage/cassandra/

Our prototype Cassandra backend for the main archive currently lacks the origin_metadata_add and origin_metadata_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)

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Migrated from T2290 (view on Phabricator)