Bring auto-accept invite logic into Synapse (#17147)

This PR ports the logic from the
[synapse_auto_accept_invite](https://github.com/matrix-org/synapse-auto-accept-invite)
module into synapse.

I went with the naive approach of injecting the "module" next to where
third party modules are currently loaded. If there is a better/preferred
way to handle this, I'm all ears. It wasn't obvious to me if there was a
better location to add this logic that would cleanly apply to all
incoming invite events.

Relies on https://github.com/element-hq/synapse/pull/17166 to fix linter
errors.

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