Add loadBalancerClass field to ListenerClass.spec

[nightkr]

Description

See stackabletech/listener-operator#285, part of stackabletech/listener-operator#288

This PR adds a two new fields to ListenerClass:

spec:
  loadBalancerClass: foo
  loadBalancerAllocateNodePorts: true

Which are mapped to Service.spec.{loadBalancerClass,allocateLoadBalancerNodePorts}, respectively. Currently these are ignored unless serviceType: LoadBalancer; in v1alpha2, serviceType should be turned into a rich enum instead, and these fields be moved into it, like so:

spec:
  serviceType:
    loadBalancer:
      loadBalancerClass: foo
      allocateNodePorts: true

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1. Changes are OpenShift compatible
2. CRD changes approved
3. Integration tests passed (for non trivial changes)

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1. Code contains useful comments
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3. Documentation added or updated
4. Changelog updated
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[nightkr]

Moving the CRD changes into RFC