Expand FieldExport to support json `path` to target arbitrary fields of matching GNKV resources

[tis-rpage]

Is your feature request related to a problem? Kustomize replacements/patches provide a means to modify specific attributes in a k8s resource via a json path. Field Exports enhanced with this capability could provide new functionality beyond targeting configMap/secrets.

For example, services of type externalName create a local DNS entry for connecting to services by a short name within the cluster. FieldExport can be enhanced to update spec.externalName of a Service so that this DNS entry is dynamically updated to match newly created AWS resources.

Describe the solution you'd like

apiVersion: services.k8s.aws/v1alpha1
kind: FieldExport
metadata:
  name: postgres-rds
spec:
  from:
    resource:
      kind: DBInstance
      name: postgres
    path: ".status.endpoint.port"
  to:
    kind: Service
    name: postgres-rds
    path: "spec.externalName"

Describe alternatives you've considered

apiVersion: apps/v1
kind: Deployment
metadata:
  name: postgres-init-rds-endpoint
spec:
  replicas: 1
  selector:
    matchLabels:
      name: postgres-init-rds-endpoint
  template:
    metadata:
      labels:
        name: postgres-init-rds-endpoint
    spec:
      serviceAccountName: postgres
      containers:
        - name: postgres-init-rds-endpoint
          image: ansible:latest
          env:
            - name: DBINSTANCE
              value: postgres
            - name: SERVICE
              value: postgres-rds
          command:
            - /bin/sh
            - -c
            - |-
              get_rds_endpoint() { kubectl get "dbinstance/${DBINSTANCE}" -o json | yq -P e .status.endpoint.address - ; }
              get_service_endpoint() { kubectl get -o yaml "service/${SERVICE}" | yq e '.spec.externalName' - ; }
              set -x
              while sleep 7 ; do
                RDS_ENDPOINT="$(get_rds_endpoint)"
                SERVICE_ENDPOINT="$(get_service_endpoint)"
                case "${RDS_ENDPOINT}" in
                  null|"${SERVICE_ENDPOINT}") :;;
                  *) kubectl patch --type merge "service/${SERVICE}" -p "{ \"spec\": { \"externalName\": \"${RDS_ENDPOINT}\" } }";;
                esac
              done