Envoy to pass hits_addend to RateLimitService

Description

The RLS v3 api 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 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.