[EngSys] Add check to verify PR author permissions

[mikeharder]

If a PR author doesn't have write access, their PR will be blocked with missing/failing checks, since workflows or pipelines may fail to run (or fail with a runtime error). Example:

https://github.com/Azure/azure-rest-api-specs-pr/pull/23592/checks?sha=7192e7b5c9aafa0dabcb8c555065f6832d0b9b3e

Perhaps we could add a simple workflow to check the users permissions, and fail with a clear error message if incorrect?