Add allowlist to db/docs files for cross-joins/fks/txns

What does this MR do and why?

As part of the Cells effort we need to classify all tables as gitlab_main_cell or gitlab_main_clusterwide. In the future these tables will be split into different databases. For now when we classify these tables we will start to get errors in CI for cross-joins, cross-transactions and cross-database foreign keys.

This MR adds an allowlist in the db/docs file per type to avoid seeing these failures once we classify the table as gitlab_main_cell. This is to to support automatically labelling hundreds of tables in bulk with gitlab_main_cell (as part of the Organization Isolation effort) . This will result in lots of pipeline failures and we want to deal with these those cross-join (etc.) fixes later. For now we just want to classify things.

You can see the example format of allowing in the db/docs/achievements.yml change which was chosen as an arbitrary example to prove that this works.

This MR was extracted from a larger MR !140420 (closed) which tried to move some duplicated logic in GitlabSchema class to the Dictionary class. This proved to be far more widespread a refactoring than I wanted because these methods are used all over the place and it required hundreds of changes to fix all the specs (which I didn't manage to finish anyway). Since this change is important and unblocks us setting gitlab_main_cell to the tables in the short term I didn't want to hold off on that refactoring.

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Related to #429172 (closed)