Add heroes and room summary fields to Sliding Sync /sync

[MadLittleMods]

Add heroes and room summary fields (joined_count, invited_count) to Sliding Sync /sync

Based on MSC3575: Sliding Sync

Dev notes

Instead of returning stripped state for heroes, we could simply return the User ID's of the heroes like Sync v2 (see m.heroes) and return the full membership event in required_state, see matrix-org/matrix-spec-proposals#3575 (comment)

We don't even have access to heroes if we're the first person on our server to be invited to a remote room over federation, see matrix-org/matrix-spec-proposals#3575 (comment)

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
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  • Start with a capital letter.
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• Code style is correct
  (run the linters)