Make injection template customizable for operator based installs.

[liamawhite]

Describe the feature request
Currently, customising the sidecar injector template requires an overlay replacing the entire injection ConfigMap, not just the template.

  components:
    pilot:
      k8s:
        overlays:
        - apiVersion: v1
          kind: ConfigMap
          name: istio-sidecar-injector
          patches:
          - path: data
            value:
			  <entire-sidecar-injector-config-map-plus-modifications>

This will mean any fields set outside of the template (config or .Values) in the Operator API will need to be changed in this overlay as well. Ideally, I would like to provide/edit the injector template only.

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

[liamawhite]

Related RFC, https://docs.google.com/document/d/1Rmp9B3DDypgMCau-YAqidx_r3qvKLZj6jUX-t22zj84/edit

cc @howardjohn

[howardjohn]

f2f6f26 essentially supports this, you can add a custom template and then change the default template to be sidecar,my-custom-template and it will be overlayed.

[istio-policy-bot]

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