Support automerge for pull requests

[bamarsha]

Is your feature request related to a problem? Please describe.
Pull requests require automatic builds to complete before merging, but these builds take a long time.

Describe the solution you'd like
It would be nice to have something like the "automerge" label that https://github.com/microsoft/qsharp-runtime has, which will automatically merge a PR once all the required status checks have passed.

Describe alternatives you've considered
N/A

Additional context
N/A