Draft: Allowlist for gitlab_schema in db/docs per table

What does this MR do and why?

There are 2 parts to this change:

1. Move a bunch of unnecessary logic out of GitlabSchema and into the Database::Dictionary. Also it moves tests to the Database::Dictionary that should have been there in the first place. This is to make the next change easier to implement.
2. Add support for configuring allow_cross_joins, allow_cross_transactions and allow_cross_foreign_keys per table in their retrospective db/docs file. This should behave equivalently to configuring the same named keys in db/gitlab_schemas where specific_tables are given. See the example configuration structure at https://gitlab.com/gitlab-org/gitlab/-/blob/f450278eb443284614e73b481c4a57476e7c0967/db/gitlab_schemas/gitlab_main_clusterwide.yaml#L17

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