Implement the new per-descriptor level hits_addend

[mathetake]

Each RateLimit request has descriptors, and each descriptor now has a dedicated hits_addend introduced in envoyproxy/envoy@cac9b87

  // Optional hits_addend for the rate limit descriptor. If set the value will override the
  // request level hits_addend.
  google.protobuf.UInt64Value hits_addend = 3;

https://github.com/envoyproxy/envoy/blob/712b73658357f97bb7b53b33b497258b5a83b1ac/api/envoy/extensions/common/ratelimit/v3/ratelimit.proto#L127-L129

For the context, previously the hits_addend was (optionally) given per-Ratelimit request, not per-descriptor.

To make this projects as a "reference implementation", we need to add support for this APi.

[mathetake]

cc @wbpcode @zirain @zhaohuabing @arkodg

[wbpcode]

See #802

[mathetake]

woah you are fantastic :D

[mathetake]

closed by #802