Caching support for External Authorization

## Feature
Add caching support for [External Auhorization](https://gateway.envoyproxy.io/docs/tasks/security/ext-auth/). Caching could be enabled on all or certain http response status code from ext auth service with configurable ttl.

## Use Case
We currently have a central auth service. Each service in our cluster talks to this for auth and also has a local cache. We wanted to remove auth handling in each service but using [External Auhorization](https://gateway.envoyproxy.io/docs/tasks/security/ext-auth/) in its current state doesn't look viable because due to lack of any caching, we will face:
- an increased latency for each api call
- increased load on central auth service

Having a cache within envoy would ideally solve this.

Found a similar issue https://github.com/envoyproxy/envoy/issues/3023. Opening this here as well because the use case to me makes more sense and seems more logical for a gateway rather than a proxy.