Define EnvoyProxy image tag in values.tmpl.yaml

*Description*:
>Describe the desired behavior, what scenario it enables and how it
would be used.

Please define EnvoyProxy image tag in values.tmpl.yaml instead of ["distroless-dev"](https://github.com/envoyproxy/gateway/blob/cbb4337459bf2adfd3c9e6c8bcffdd3ba4cdcffa/charts/gateway-helm/templates/_helpers.tpl#L171):

`{{- $imageTag := default "distroless-dev" $repositoryParts._1 -}}`

and [hardcoded](https://github.com/envoyproxy/gateway/blob/cbb4337459bf2adfd3c9e6c8bcffdd3ba4cdcffa/api/v1alpha1/shared_types.go#L29)

`DefaultEnvoyProxyImage = "docker.io/envoyproxy/envoy:distroless-dev"`