build: envoy gateway support (helm) [WPB-24493]

[jschaul]

WPB-24493 add gateway support to webapp ingress (part of webapp helm chart)

In general we wish to switch from ingress-nginx to envoy-gateway. This
has already been added as an option when the webapp is installed as part of the
wire-server bundle (see wireapp/wire-server#5150
and related PRs). But when using this chart standalone, it also needs
to be added here.

Ticket: https://wearezeta.atlassian.net/browse/WPB-24493

• Regression deployment: The default nginx-ingress helm chart is being deployed through argo
  https://argocd.apps-staging.ops.zinfra.io/applications/argo-cd/webapp-pr-21860 using these overrides and is available under https://webapp-pr-21860.apps-staging.wire.link
  • the only change is from ingress: enabled: true to routing: enabled: true / mode: nginx but I believe the standalone webapp is not being used/deployed anywhere except through argo manifests (which is updated alongside this PR)
• Envoy based deployment: Using these overrides this gets deployed with Argo and is available under WIP and hinges on extra infra created in the cluster