MSC4242: State DAGs (fed client)

[kegsay]

Split out from #19425

• Add federation support for MSC4242 fields in /send_join and /get_missing_events
• Move the gate preventing joining MSC4242 rooms higher in the call stack

Part of a series of 5x PRs to land the federation part of MSC4242 (storage, fedclient, serving, inbound-joins, inbound-pulls).

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.
  • 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)