Evnoy ExtAuth GRPC re-connecting

*Description*:
> It seems that envoy creates new connection to ExtAuth GRPC every time it needs to be invoked, which is not ideal for latency.

*Repro steps*:
> Install Envoy and connect it to GRPC service for ExtAuth. I can't really show our ExtAuth but [this](https://github.com/salrashid123/envoy_external_authz) should be sufficient.

*Environment*:
> K8S deployment of EnvoyGateway + K8S deployment of grpc service

*Config*
```
extAuth:
failOpen: true
grpc:
backendRefs:
- group: ''
kind: Service
name: ...
port: 8080
```

*Note*
> Maybe I'm not right, but i wasn't able to find that info in docs