Restrict which users can add certain labels to PRs

[mikeharder]

It has been suggested to restrict which users can add certain labels to PRs. For example, some breaking change labels should only be applied by the breaking change board.

Implementation options:

• GitHub Action that triggers on pull_request:labeled event, checks user, and removes label if user is not allowed. Simpler, but could be circumvented, say if the action fails to run once when the label was added.
• GitHub Action that uses timeline events API (https://octokit.github.io/rest.js/v18/#issues-list-events-for-timeline) to block PRs, if the most recent timeline event for a label was added by a disallowed user. Slightly more complex to implement, but more reliable since it would block until it confirms the label was added by the correct user.

[mikekistler]

There is already an issue open for this in the azure-sdk-tools repo:

Azure/azure-sdk-tools#6035