feature: Allow overriding the backend name and port in keycloakx chart

[hpdvanwyk]

This is useful for overriding the backend of certain paths to not go to Keycloak.
One use case is when using Keycloak with the https://github.com/aerogear/keycloak-metrics-spi plugin along with aws-load-balancer-controller. This allow using the "alb.ingress.kubernetes.io/actions.${action-name}" annotation to make in-realm metrics endpoints 404 instead of exposing them on the public internet. This would otherwise be annoying to achieve since this plugin puts metrics on <base url>/realms/<realm>/metrics.

For example this allows creating the following ingress object which will 404 on all /realms//metrics paths while allowing everything else under /realms/:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: release-name-keycloakx
  namespace: default
  annotations:
    alb.ingress.kubernetes.io/actions.response-metrics: "{\"type\":\"fixed-response\",\"fixedResponseConfig\":{\"statusCode\":\"404\"}}\n"
spec:
  ingressClassName: alb
  tls:
    - hosts:
        - "${host}"
  rules:
    - host: "${host}"
      http:
        paths:
          - path: "/auth/realms/*/metrics"
            pathType: ImplementationSpecific
            backend:
              service:
                name: response-metrics
                port:
                  name: use-annotation
          - path: "/auth/realms/*"
            pathType: ImplementationSpecific
            backend:
              service:
                name: release-name-keycloakx-http
                port:
                  name: http

with the values.yaml snippet:

  rules:
    - host: "${host}"
      paths:
        - path: '{{ tpl .Values.http.relativePath $ | trimSuffix "/" }}/realms/*/metrics'
          pathType: ImplementationSpecific
          backendOverride:
            name: response-metrics
            port: use-annotation
        - path: '{{ tpl .Values.http.relativePath $ | trimSuffix "/" }}/realms/*'
          pathType: ImplementationSpecific