[owners] Support reviewerPool of users and/or teams in place of reviewerTeam property

[rcebulko]

Closes #1283
Requires an update to amphtml's root OWNERS file, but this should not fail any PRs on that repo in the interim unless they trigger OWNERS build target. Will require pushing an updated deploy tag to trigger a build.

Before: reviewerTeam allowed only in repo root OWNERS file, must be a string representing a team
Now: reviewerPool allowed only in repo root `OWNERShi file, must be an array of strings representing teams or usernames

[rcebulko]

@rsimha Can you make the necessary changes on the amphtml side and I'll handle re-deploying this?

[rsimha]

Thanks for the quick fix! LGTM with a few nits.

Happy to help coordinate this change with the one needed in amphtml.

[rsimha]

@rsimha Can you make the necessary changes on the amphtml side and I'll handle re-deploying this?

SG. I've sent out ampproject/amphtml#34176 for review. I can help coordinate the merging of it with this PR.

[rcebulko]

@rsimha FYI CircleCI migration may have introduced a bug in built target detection:

Locally I get the correct set of build targets, suggesting it may be due to the git merge-base main HEAD using the PR merge commit instead of the PR head commit.

[rsimha]

Locally I get the correct set of build targets, suggesting it may be due to the git merge-base main HEAD using the PR merge commit instead of the PR head commit.

Could be due to GH actions. Thanks for the heads up. Will address this via a separate fix.

Edit: Addressed in #1307, confirmed in #1307 (comment).