add support for the msc4143 rtc_foci well-known directive

[jessebot]

• Fixes matrix_rtc details not included in client well-known #19484
• Fixes Synapse 1.150.0 does not expose MatrixRTC transports endpoint despite msc4143_enabled=true #19652

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)

[jessebot]

I have limited time to work on this, but if there's something obvious I can do to improve it, or more testing I can do outside of the linter, please let me know 🙏