fix(ci): derive requested teams from CODEOWNERS and detect existing PR assignees in assign-reviewers (#9703)

Fixes reviewer auto-assignment in
`.github/workflows/assign-reviewers.yml` without requiring organization
team token permissions:

### Fixes ###
- Add `@quirogas` to `# team_members:`.
- Parse file-to-team rules directly from `.github/CODEOWNERS` instead of
`pulls.listRequestedReviewers` (which returns `teams: []` under
`GITHUB_TOKEN` and previously caused `hasDefaultTeam` to evaluate to
`false`, skipping removal of `cloud-sdk-nodejs-team`).
- Check human `requested_reviewers`, `assignees`, and `reviews`
(`hasHumanReviewerOrAssignee`), and trigger on `review_requested` and
`assigned` events so `cloud-sdk-nodejs-team` is reliably removed
whenever another reviewer is assigned.

Author: shivanee-p

.github/CODEOWNERS

@@ -6,7 +6,7 @@
 
 
 # The cloud sdk nodejs team is the default owner for nodejs repositories.
-# team_members: @bshaffer @feywind @danieljbruce @shivanee-p
+# team_members: @bshaffer @feywind @danieljbruce @shivanee-p @quirogas
 *     @googleapis/cloud-sdk-nodejs-team
 
 # Generated GAPIC packages, core libraries, and documentation are strictly owned by the Node.js SDK team