Deprecating legacy ratelimit proto
Fixes #14
In order to not break any clients using the legacy ratelimit.proto, Ratelimit is going to support both handlers: the one for the data-plane-api service definition (rls) and the legacy service definition (ratelimit.proto).
The legacy handler will accept a request, translate it into a data-plane-api proto request and use the rest of the codebase as if the new handler had been called. This will incur a perf penalty for clients using the legacy proto.