WIP: Support for profile updates over Sliding Sync (MSC4262)

[jaywink]

This PR implements support for profile updates over Sliding Sync: matrix-org/matrix-spec-proposals#4262.

This PR depends heavily on anoa/msc4429 and is currently in WIP status due to various missing features.

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.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
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• Code style is correct (run the linters)