Add support for specifying extra manifests via Helm for emissary-ingress chart

[oponomarov-tu]

Please describe your use case / problem.

It would be very beneficial to supply extraManifests value to the emissary-ingress Helm chart that would render extra manifests as a part of a single Helm chart deployment.

Describe the solution you'd like

An extra variable to hold a list of arbitrary Kubernetes manifests, e.g.

## Extra manifests to deploy as an array
extraManifests: []
  # - apiVersion: elbv2.k8s.aws/v1beta1
  #   kind: TargetGroupBinding
  #   metadata:
  #     name: internal-emissary-ingress
  #   spec:
  #     serviceRef:
  #       name: internal-emissary-ingress
  #       port: 80
  #     targetGroupARN: <target group arn>

Describe alternatives you've considered

1. Building a Helm chart wrapper to include emissary-ingress Helm chart as a subchart.
2. Using Kustomize with overlays.
3. Deploying the extra manifests in a separate Helm chart / Kustomize overlay.

The (1) & (2) are hard to maintain and scale, the (3) introduces the dependency between deployments which requires putting the stuff in correct order.

Additional context

This has been a normal practise and a lot of OSS Helm charts support such options these days, e.g.

• https://github.com/prometheus-community/helm-charts/blob/ff10663d1c9fe327afccbf70698649621761079e/charts/kube-prometheus-stack/values.yaml#L4727
• https://github.com/external-secrets/external-secrets/blob/22c1af40e09d20b1bcedf2be28cb7022ba67a702/deploy/charts/external-secrets/values.yaml#L129

I'm willing to implement this feature.