Create assignment bot based on PullRequest Label change webhook

[ScottSuarez]

Background

Assigning a reviewer is tricky for a couple reasons.

• The information for /who/ to assign is in several places
• A PR cannot receive an assignment from the full list if you are on the review rotation. Reviews in this pool manually assign.
• When they manually assign they use the terraform-team group which both omits hashicorp folk /and/ is unaware of when someone is out of office.

What kind of contribution is this issue about?

• general contributions

Details

Introduce a bot based on a webhook that will auto assign a reviewer and take into context the scenarios not covered in the background above.

[rileykarson]

Note: This seems to be specific to maintainer assignment