Draft: Resolve "Spike - Proof egress gateway virtual service templating"

General MR

Summary

Develop templating pattern for egress gateway routing to subsequently be applied to all other bigbang packages.

Requires the following gateway config to exist, currently being manually kubectl applied:

apiVersion: networking.istio.io/v1beta1 
kind: Gateway
metadata:
  name: istio-egressgateway
  namespace: istio-system
spec:
  selector:
    istio: egressgateway
  servers:
    - port:
        number: 443
        name: tls
        protocol: TLS
      hosts:
        - "*"
      tls:
        mode: PASSTHROUGH

Relevant logs/screenshots

(Include any relevant logs/screenshots)

Linked Issue

#132 (closed)

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