Envoy to pass hits_addend to RateLimitService

## Description
The [RLS v3 api](https://www.envoyproxy.io/docs/envoy/latest/api-v3/service/ratelimit/v3/rls.proto) describes the RateLimitService as able to injest a hits_addend field to determine number of tokens to use for the rate limiting request.
Envoy should provide a method for extracting a value from a request header (or some other method) to populate this method on a _per request_ basis. If hits_addend is only static, then it is effectively the same as modifying the ratelimit.

### Use case
In the [HTTP Rate Limit Filter](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/rate_limit_filter#config-http-filters-rate-limit) allow for a configuration of a request header containing an integer hits_addend value to send with the rate limit request, allowing for greater configurability of rate limiting capabilities.