Implement custom error/response options based on different backend response codes

[josh-pritchard]

Potentially leverage https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/filters/http/custom_response/v3/custom_response.proto#envoy-v3-api-msg-extensions-filters-http-custom-response-v3-customresponse to accomplish this.

[spline2hg]

@josh-pritchard can i work on this?

[pradhyum6144]

Hi @josh-pritchard, I'm also interested in working on this feature. i am already working on this code base so if you would assign this issue to me i will solve this issue within hrs.

Thank you