Expose provider in the API tags response

[AetherUnbound]

Fixes

Fixes #4273 by @AetherUnbound

Description

This PR exposes the provider field within the tags attribute that is present in the database under the key unstable__provider. The core of this change is in media_serializers.py, most of the other changes in this PR revolve around making sure the field shows up in tests and documentation.

One thing that was unexpected was having to set allow_null=True on the serializer. It seems like there are some cases where we're converting results directly from Elasticsearch into serialized results rather than retrieving the full data from the database. We could alter the ingestion server to ingest this new field, but I don't think it's necessary (and I think we can leave this as nullable here, given the explanation I've provided).

Testing Instructions

Tests should pass! You can also do the following:

1. just down -v
2. just api/init
3. Visit http://localhost:50280/v1/images/ and see that each of the tags arrays also sends an unstable__provider key
4. Visit http://localhost:50280/v1/images/?q=crim and observe a result that has tags from multiple providers

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (just catalog/generate-docs for catalog
  PRs) or the media properties generator (just catalog/generate-docs media-props
  for the catalog or just api/generate-docs for the API) where applicable.

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[obulat]

Nice that the change is so small! Thank you for the explanation on allow_null. I wonder if this is only this way in the local data, or in prod, too?

[AetherUnbound]

Thank you for the explanation on allow_null. I wonder if this is only this way in the local data, or in prod, too?

Because it has to do with what we index into Elasticsearch, I assume it would affect production as well. The local data is "complete" in the sense that all of the tags have a provider in the actual database. If anything, I would imagine production to be less compliant on that front!

[zackkrida]

Looks great!