test-infra: Prow support for triggering jobs based on tag changes

It would be useful to the cluster-registry if Prow could support triggering jobs based on GitHub CreateEvent"s.

Concrete use case: I would like to be able to do cluster-registry releases without having to run tools locally on my machine, based on tags created by a GitHub release. If Prow can respond to GitHub CreateEvents, then creating the tag can run the release process/upload binaries/etc. without requiring the person. This allows releases to be done by people who may not have write access to the GCP project that hosts the binaries, simply by creating a

Subnote: it might be useful to know if a tag is verified, and to only start a release based on a verified, signed tag. cf https://help.github.com/articles/signing-tags-using-gpg/