Support additional containers in gateway Helm chart

[gjacquet]

Describe the feature request

There are some use cases where it would be useful to run a sidecar container along with the gateway main container.

Gateways chart supports it (https://github.com/istio/istio/blob/master/manifests/charts/gateways/istio-ingress/values.yaml#L72) however the gateway chart to deploy a single gateway does not allow defining any additional containers.

Describe alternatives you've considered

While there are plenty of other way to get this done (forking the chart, using an admission controller etc), adding the option in this chart would be the most convenient.

Affected product area (please put an X in all that apply)

[ ] Docs
[x] Installation
[ ] Networking
[ ] Performance and Scalability
[ ] Extensions and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience
[ ] Developer Infrastructure

Affected features (please put an X in all that apply)

[ ] Multi Cluster
[ ] Virtual Machine
[ ] Multi Control Plane

Additional context

[howardjohn]

See https://docs.solo.io/gloo-edge/latest/installation/gateway/kubernetes/helm_advanced/; I don't think we can reasonable expose every single possible API field in the gateway.