Add metadata to our publish task for Tekton Chains to observe & sign
Add metadata to our publish task for Tekton Chains to observe & sign

This commit adds an annotation to indicate that build provenance should be
generated and an `IMAGES` result composed of a comma-separated list of
imageNames+digest to be signed.

This change is based on
https://github.com/tektoncd/chains/blob/main/release/publish.yaml and
https://github.com/tektoncd/plumbing/blob/main/docs/signing.md

tektoncd/pipeline,tektoncd/pipeline