Decide where/how to store extrinsinc metadata

Before implementing the metadata spec, we need to know where to implement it in; so we need to decide where to store it.

I see three options:

1. in the main swh-storage, which may cause some issues scaling
2. implemented in swh-storage, but running on a different postgres/cassandra cluster, which might be confusing
3. implemented in a new component entirely, which adds some complexity and code duplication with swh-storage if we decide to use postgres or cassandra

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Migrated from T2343 (view on Phabricator)