Add template function to hash a resource in the same manifest

[NiklasRosenstein]

This would be very useful to generate a typical "config hash" label that can be used to rotate Pods in a deployment when another resource, usually a Secret or ConfigMap, changes.

apiVersion: apps/v1
kind: Deployment
# ...
spec:
  template:
    metadata:
      labels:
        confighash: ${{ hash(local_lookup("v1", "Secret", "mysecret", "namespace").data) }}
  # ...