workflows: Add a simple pull request subscription workflow (#64913)

This new workflow will make it possible for people to subscribe to pull
requests based on the labels that are added. Labels will be added
automatically to the pull requests based on the modified files and each
label will be associated with a GitHub team that will be notified when
the label is added.

See
https://discourse.llvm.org/t/changes-to-pull-request-subscription-system/73296

Author: tstellar