Add support for bisecting JAX workflows

[nitins17]

This PR adds support for bisecting JAX-style workflows. JAX's continuous workflows clone openxla/xla at HEAD. Re-running a failed workflow would require pinning both XLA and JAX commits to the original state.

This is solved by:

1. Detecting when a bisected project (e.g., jax-ml/jax) requires dependency pinning.
2. Looking up the commit timestamp of the JAX commit being tested.
3. Querying the dependency repo (openxla/xla) for the last commit before that timestamp.
4. Injecting that historical dependency SHA into the workflow inputs (e.g., xla-commit).

Changes
culprit_finder.py

• Added PROJECT_CONFIG to map repositories to their external dependencies and input variable names.
• Updated _test_commit to fetch the commit date and resolve the correct dependency SHA before triggering the workflow.

github_client.py

• Added get_commit(sha) and get_last_commit_before(repo, date)