BCR bazel compatibility test: Add generate_report.py (#2091)

This will be run in the last step for a
https://buildkite.com/bazel/bcr-bazel-compatibility-test build without
`USE_BAZELISK_MIGRATE` and generate a report in markdown that can be
used for filing a GitHub issue report for broken modules.

e.g.:
<img width="685" alt="image"
src="https://github.com/user-attachments/assets/3e888dd0-601c-4b38-bce0-047eb80be9bb">
2 files changed