Sliding Sync: Lazy-loading room members on incremental sync

[MadLittleMods]

Lazy-loading room members on incremental sync

Fix #17804

This is currently just a first pass and doesn't remember which membership events that it has sent down before. That functionality requires more complexity on top of this change and #17785 which I'd rather split out to another PR.

Pull Request Checklist

• Pull request is based on the develop branch
  • Wait for Correctly handle changes to required state config in sliding sync #17785 to be merged
• 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.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[MadLittleMods]

Closing in favor of #17809