Add room_type filter on /_synapse/admin/v1/rooms API

[velikopter]

Adds a room type query parameter to /_synapse/admin/v1/rooms.

Spawning from https://matrix.to/#/!LpivaKUdewaGfawMoR:etke.cc/$gtCQG4A8soWDtk9IQdjRu8hrmR-aForMiziPvxJpVFc?via=etke.cc&via=nhjkl.com&via=unredacted.org

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)

[reivilibre]

This seems useful. Just one suggestion, but I'm not massively fussed either way