publish GitHub action to apply a holobranch to a k8s cluster

[themightychris]

Publish a javascript action for deploying a holobranch to a cluster

Potential inputs:

• holobranch: Name of holobranch containing Kubernetes content
• kubeconfig: Contents of kubeconfig file to use for authentication
• ref: Commit-ish to read holobranch from (defaults to action context)
• diff-since: Commit-ish of the last deployed source to compute a diff from to apply a changeset to the cluster
  • only way to have k8s resources get deleted when they're removed from git
  • defaults to unset, so entire content tree gets applied recursively
  • might typically be set to the previous head github action variable in pull request events
• namespace: Namespace to apply manifests within, defaults to whatever is in kubeconfig

If the action is triggered from a PR, it should post comments back to the PR throughout the deploy process, documenting when projection starts, when deployment starts, each cohort of resources and their results, and the final result

[themightychris]

Maybe provide a second action for validating a holobranch in a PR check. Optionally given a kubeconfig to diff against cluster and post back to triggering PR. Would be used as a PR check

[themightychris]

Also: ensure STDERR output gets captured and reported to the PR separately (currently seems to be dropped from the PR comment)