Consider assigning random TW reviewer from CODEOWNERS

Right now, if a pattern has several code owners, we'd assign all of them in the AutomatedReviewRequestDoc processor.

What we could do instead is to:

1. Select code owners that are available (based on roulette data)
2. Pick a random code owner from the list, and assign them

This was suggested from gitlab-org/gitlab-development-kit!2592 (comment 1047805601).