Enforce a 25 MiB Git blob size limit in GitHub Actions and pre-commit (#2985)

Add `.github/workflows/check-file-sizes.yml` and
`.github/scripts/check_file_sizes.py` to reject Git blobs larger than 25
MiB (`26,214,400` bytes) in `HEAD` or across commits in the pull request
or push commit range due to limitations of an internal clone of this
repository.

Also extend `.githooks/pre-commit` to enforce the same staged blob size
limit locally, and add unit tests in
`.github/scripts/check_file_sizes_test.py`.

context: b/580911465