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)