Add ability to rate limit messages per room

[x48115]

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

[x48115]

For reference here is implementation for per room rate limit joins:
#12710