Add initial support for EgressGateways

For https://repo1.dso.mil/platform-one/big-bang/apps/core/istio-controlplane/-/issues/22

This enables "native" support for egress gateways but leaves them off by default.

To test deploy with:

istio:
  enabled: true
  git:
    tag: null
    branch: egress-gateways
  values:
    egressGateways:
      istio-egressgateway:
        enabled: true

You will need to use MetalLB since you'll have two LB services (ingress and egress gateway). To validate it worked I went through the lab here and validated traffic was properly passing through the egress gateway. NOTE: If you follow the steps there to turn the mesh config to REGISTRY_ONLY it will break a lot of the BB functionality since things will be unable to talk to each other.