Add MatrixRTC backend/services discovery endpoint

[fkwp]

This PR was taken over by @anoadragon453.

This PR adds a new, authenticated client discovery endpoint for MatrixRTC transports that the homeserver supports. Previously, clients would check the homeserver's .well-known/matrix/client file. This had several issues:

• Anyone could discover details on how to connect to the SFU, potentially exposing that service to requests from unwanted parties. (In the future, this endpoint may contain a user-specific secret to auth to the SFU.)
• In practice, several enterprise organisations have hit bureaucratic roadblocks in getting approval to update a .well-known/matrix/client file under their top-level domain (i.e. bigcorp.com). Being able to only update Synapse config and restart the service minimises the time it takes to change details in large organisations.

There is an internal Google Doc containing the WIP updates to MSC4143. This PR implements those changes.

---

Recommended to review commit-by-commit.

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)