Simple rate limit

[pedropregueiro]

A lot of folks we talked to re: chat building have raised the issue of bot spam and how rate limiting is crucial from the get-go to avoid downtime caused by this.

I don't think we need something super complex to start. This middleware is referenced from Starlette's docs: https://github.com/abersheeran/asgi-ratelimit and seems pretty customizable.