Add bcr-pr-review-approver (#1887)

This PR implements a GitHub Action that helps review and approve BCR
PRs.

- The GitHub Action should be triggered on schedule instead of by
`pull_request_review` event, because there is no way to obtain a GitHub
token that has write permissions if the PR is from a forked repo.
(Related https://github.com/orgs/community/discussions/26651)
- The GitHub Action goes throw all open PRs to review them one by one.
- The GitHub Action approves a PR if the latest non-merge commit is
approved by at least one maintainer for each modified module.

Auto-merging the PR is for now disabled until we confirm this is fine,
instead, `@bazelbuild/bcr-maintainers` will be notified to take a final
look.
531 files changed