When discoveries change, regenerate corresponding samples into new PR

[vchudnov-g]

In order for us to update samples more proactively, let's implement the following:

After syncing new Discoveries, our nightly job should re-generate samples

• for any Discoveries that have changed, or
• if any of the code in this repo has changed.

If those new samples are different than the corresponding existing samples, the new samples should go into a PR for review by the owners of this repo.

The owners of this repo will thus be automatically notified when new samples need to be published. After approving and merging the PR, we can then pull it into our local workspaces and push the new samples to GCS where they can be visible to the world.