Systematic validation of Synapse's config

This issue has been migrated from [#12651](https://github.com/matrix-org/synapse/issues/12651).

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I'd love to see something does all of our config key checking and typechecking at startup time. Ideally this would give us useful type annotations which could flow through the rest of the program; but I think we should optimise for something which produces good error messages for the server owner.