Automate Reviewer Assignment

[damccorm]

Right now, we don't have a well defined automated system for staying on top of pull request reviews - we rely on contributors being able to find the correct OWNERS file and committers manually triaging/calling attention to old pull requests. We should add automation driven by GitHub Actions to automatically round robin new PR reviews to a set of contributors, thus balancing the load.

 

Design doc here - https://docs.google.com/document/d/1FhRPRD6VXkYlLAPhNfZB7y2Yese2FCWBzjx67d3TjBo/edit#

Imported from Jira BEAM-13925. Original Jira may contain additional context.
Reported by: damccorm.

[damccorm]

The following subtask(s) are associated with this issue: #21418, #21419, #21420, #21421

[damccorm]

#22258 should be fixed before this is considered done