Add redis config option so that I can use helm

[daniel-white]

Description:
I'm going to be using global rate limiting. Right now the steps call for a ConfigMap to patch the envoy-gateway.yaml file to get the redis backend. With setting this file and starting a restart of the gateway deployment, this is not easily managed. I'd imagine that we have a resource that we can easily set the redis url without that dance.

[optional Relevant Links:]
Enabling steps: https://gateway.envoyproxy.io/docs/tasks/traffic/global-rate-limit/#enable-global-rate-limit-in-envoy-gateway