Allow purely random assignment

Feature Request:
We'd like to bypass most of the algorithm and allow for a purely random assignment.

Use case:
Our code base has been heavily modified by one person. This person usually gets assigned most PRs for review. However we want new employees to get a good understanding of the project so we want them to get assigned PR review for code they have not yet contributed to.

No files, tests, or entry points are named. Start by locating the reviewer-assignment algorithm and its existing assignment configuration or tests, then determine how a purely random mode should be selected and bounded. Done means the requested mode can bypass most of the algorithm and assign reviews randomly, with coverage for that behavior.

Written by the indexing model from the issue text.