[GitHub] Add greeting comment to opened PRs from new contributors (#72384)

This includes some commonly needed information like how to add
reviewers.

This is implemented as a job before the labeler, so that on a new PR the
comment is added before there are any subscribers and only the author
gets a nofitication.

The labeler job depends on the greeter having run or having been
skipped. So if the PR wasn't just opened, or it's from a regular
contributor, the labeling still happens.

But we can be sure that when a greeting comment is left, it's the very
first thing we do.