explore adding `labeler` action

https://github.com/actions/labeler would help w/ PR reviews as we can auto label PRs based upon paths changed, so we can very easily see which Go modules will get updated in that PR.

Begin by reviewing the repository's existing GitHub Actions configuration and the linked actions/labeler documentation. Define path-based labels for the Go modules so pull requests receive labels indicating which modules they update, then verify the labeling behavior on a representative pull request.

Written by the indexing model from the issue text.