conditional retry condition in retry policy

Description:

https://github.com/envoyproxy/envoy/blob/abb1f969fe131c7a47da5c1ad1577be142e1ea8a/api/envoy/config/route/v3/route_components.proto#L884

defines retry policy for HTTP request. It doesn't support configure different retry policies for different HTTP methods. Users might want to retry on connection failure for all methods but 5xx for idempotent methods only. I think the retry policy should provide such function to allow configure different retry policies for different HTTP methods. More generally, the retry policy should be able to support conditional retry condition.

Relevant Links: https://github.com/istio/istio/issues/13851