Ratelimit Static Limit Override

Added a static :ref:rate_limit <envoy_v3_api_field_config.route.v3.RateLimit.Override.rate_limit> source to the rate limit :ref:limit <envoy_v3_api_field_config.route.v3.RateLimit.limit> override. This allows a fixed requests_per_unit/unit limit override to be attached to a descriptor directly from configuration, without requiring the value to be sourced from dynamic_metadata.