Enable a way to trigger downstream build and test jobs based on a PR

[dramdass]

This may involve a maintainer running an internal Jenkins job with a PR ref or commit SHA on a topic branch.

[janitha]

The most ideal solution would be to expose VMware internal CI/CD system webhooks to github, but that's going to be a hurdle with IT/Security.

While not instantaneous, another option is for VMware internal CI/CD to poll this project periodically/frequently for pending PRs, and do the necessary builds/tests, and then post back status on the PR.

Also, seems like we should have some gating mechanism before build/test are triggered within the VMware CI/CD (this was a point raised by the security folks), since the PR's change may contain malicious code that would run as part of the build/test jobs. One idea would to be introduce some sort of tag/label that can be set on the PR by a maintainer/reviewer.