feat: Add overrides for ingress backend service name and port

[elkh510]

This change adds support for overriding the ingress backend service name and port via the serviceName and servicePort values while keeping the existing defaults.

These overrides are required to support AWS ALB “action” annotations such as:

alb.ingress.kubernetes.io/actions.block-master-account: |
  {
    "type": "fixed-response",
    "fixedResponseConfig": {
      "contentType": "text/plain",
      "statusCode": "403",
      "messageBody": "Access Forbidden"
    }
  }

alb.ingress.kubernetes.io/actions.redirect-root: |
  {
    "type": "redirect",
    "redirectConfig": {
      "statusCode": "HTTP_301",
      "path": "/realms/shared/account",
      "query": "#{query}"
    }
  }

In these cases, the ingress backend must reference a logical action name instead of a real Kubernetes Service:

paths:
  - path: /realms/master/account/
    pathType: Prefix
    backend:
      service:
        name: block-master-account
        port:
          name: use-annotation

Without the ability to override serviceName and servicePort, such ALB action backends cannot be expressed through the chart values.

[elkh510]

@stefangries hi
Could you spare a few minutes for this PR?