Initial database_setup RPC functions

[mathemancer]

Fixes #3654
Fixes #3655
Fixes #3656

This adds database_setup.create_new, database_setup.connect_existing, and databases.list RPC functions.

Technical details

Note that while this PR doesn't actually break anything, it does remove some functionality from the connections RPC functions. I think that's okay, since we'll be replacing those.

All RPC functions are documented, and the documentation should be sufficient to understand them. If not, we need to improve the docs before merging this PR.

Screenshots

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[pavish]

Looks good to me!

[Anish9901]

Looks great overall @mathemancer, I've pointed out a minor bug please see the specific comment below.

[Anish9901]

This throws up an IntegrityError when we try to add the existing credentials via the API, I think we ought to use get_or_create here as well, to ensure that we allow users to reinstall mathesar even if the database gets deleted outside of mathesar.

[mathemancer]

I'm okay with that change.

[mathemancer]

@Anish9901 Ready for re-review

[Anish9901]

Looks great @mathemancer, Thanks!