Support returning custom HTTP response bodies in the ratelimit filter

[esmet]

Title: Support returning custom HTTP response bodies in the ratelimit filter

Description:
When using the ratelimit HTTP filter, Envoy should be able to return a custom response body provided by the external, gRPC ratelimit service. This behavior would be similar to ext_authz.

[optional Relevant Links:]
External authorization supports sending a response body from the auth service to the downstream client:

envoy/api/envoy/service/auth/v3/external_auth.proto

Lines 57 to 59 in 6a8785c

	// This field allows the authorization service to send a response body data
	// to the downstream client.
	string body = 3;