Add a test which ensures that new tables get PRIMARY KEYs.

This issue has been migrated from [#16648](https://github.com/matrix-org/synapse/pull/16648).

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We have a history, even recently, of not using PRIMARY KEYs and many of the cases seem tragically just because someone was unaware that it is good practice / maybe cribbed off earlier schema deltas. In many cases a `UNIQUE` index was created instead when that could've been a `PRIMARY KEY`.

This PR adds a test that flags up missing PRIMARY KEYs.

Base: `develop`

Original commit schedule, with full messages:

1. Add a PRIMARY KEY lint