[ENH] Add a simple rate limiter for async.

[rescrv]

This adds a rate limiter at the level of the fast api async code. The
reason for doing this is that by the time stuff gets to the pool,
there's a queue. We want to try and make the rate limiter async so that
we can avoid said queue.

[github-actions]

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[drewkim]

Nit: I'm unsure we need to wrap get_user_identity, create_database, get_database, create_tenant, get_tenant, list_databases, list_collections, count_collections, or create_collection. These operations don't take in a collection which our rate limiting is dependent on. Not blocking.