Add ability to rate limit messages per room

This issue has been migrated from [#16021](https://github.com/matrix-org/synapse/issues/16021).

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**Description:**

I am building an RFQ (request for quotes) system with Matrix/Synapse.

I'd like to have the ability to set per room rate limits.

- "Request for quotes" should be rate limited (because requesting a quote causes quote responders to do work)
- "Response to quote" should not be rate limited (because a quote responder must be able to respond to many simultaneous quotes)

What is the best way to accomplish this with Matrix today?

I see matrix supports `rc_invites` `per_room`, but not `rc_message` `per_room`

Start by reviewing how Synapse currently handles the rc_invites per_room setting and the existing rc_message rate limit configuration. Define how per-room message limits and message-type exceptions should be configured, then confirm that the resulting behavior meets the request for rate-limited quote requests and unrestricted quote responses.

Written by the indexing model from the issue text.