Implement overrides to allow users to set Service fields

[adwk67]

The listener class spec does not allow for any Service-related fields to be set by the user, other than the ones explicitly defined. The full spec is here.

[NickLarsenNZ]

I was going to raise a decision for this, because there are many fields that we might want to allow configuration for (eg: spec.loadBalancerClass).

I think instead of having:

apiVersion: listeners.stackable.tech/v1alpha1
kind: ListenerClass
metadata:
  name: aws-ec2-nlb
spec:
  loadBalancerAllocateNodePorts: false
  preferredAddressType: HostnameConservative
  serviceAnnotations:
    service.beta.kubernetes.io/aws-load-balancer-scheme: internet-facing
  serviceExternalTrafficPolicy: Local
  serviceType: LoadBalancer
  # See: https://docs.aws.amazon.com/eks/latest/userguide/auto-configure-nlb.html#_sample_service
  serviceLoadBalancerClass: eks.amazonaws.com/nlb # new field

it could be more like:

apiVersion: listeners.stackable.tech/v1alpha2 # 👈 
kind: ListenerClass
metadata:
  name: aws-ec2-nlb
spec:
  preferredAddressType: HostnameConservative
  serviceOverrides:
    metadata:
      annotations:
        service.beta.kubernetes.io/aws-load-balancer-scheme: internet-facing
    spec:
      allocateLoadBalancerNodePorts: false
      externalTrafficPolicy: Local
      serviceType: LoadBalancer
      # See: https://docs.aws.amazon.com/eks/latest/userguide/auto-configure-nlb.html#_sample_service
      loadBalancerClass: eks.amazonaws.com/nlb

Maybe we need to be careful about some of the fields, such as selectors. They would be purely defined by the operator.

[NickLarsenNZ]

More specific labels annotations are sometimes needed on the specific Listener produced Service instead of the the whole class.

For example, the external-dns DNS name, or AWS NLB/ALB Certificate binding:

external-dns.alpha.kubernetes.io/hostname: app.example.com
service.beta.kubernetes.io/aws-load-balancer-ssl-cert: arn:aws:acm:xx-central-1:123456789012:certificate/a81bc81b-dead-4e5d-abff-90865d1e13b1

[NickLarsenNZ]

I have created a related issue but it is specific to Service annotations: #331.

Also, see this issue about a general restructure of ListenerClass fields: #330