Add admin API for fetching room reports

[turt2live]

Very much a copy/paste of the event reporting API: https://element-hq.github.io/synapse/latest/admin_api/event_reports.html

Differences:

• No sender or sender filtering - rooms don't have senders.
• No get individual report detail API - there are other APIs for getting room details (which are the relevant parts of a report anyway).
• No delete individual report API - just isn't needed at this stage. A future PR/contributor can add one.
• No score - there isn't any.
• No event_id - there isn't any.

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]

@turt2live the trial unit tests are failing FWIW

[turt2live]

I'm not seeing errors which look related to this PR. Is there somewhere else I should be looking?

[anoadragon453]

A few minor things, but on the whole looks good! Tests are failing due to this PR though, see below.

[turt2live]

Replaced by #19648