[rag evals][1/n] refactor base scoring fn & data schema check

[yanxi0830]

What does this PR do?

• Refactor BaseScoringFn to be just a minimal interface, add new RegistrableBaseScoring
• Refactor data schema check
  • To separately evaluate retrieval component in RAG, we will have scoring functions needing "context" column additionally.
• Refactor braintrust eval (more scoring fn added & tested in following PR)

Test Plan

pytest -v -s -m llm_as_judge_scoring_together_inference scoring/test_scoring.py --judge-model meta-llama/Llama-3.2-3B-Instruct
pytest -v -s -m basic_scoring_together_inference scoring/test_scoring.py
pytest -v -s -m braintrust_scoring_together_inference scoring/test_scoring.py

pytest -v -s -m meta_reference_eval_together_inference eval/test_eval.py
pytest -v -s -m meta_reference_eval_together_inference_huggingface_datasetio eval/test_eval.py

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Ran pre-commit to handle lint / formatting issues.
• [ ] Read the contributor guideline, Pull Request section?
• [ ] Updated relevant documentation.
• [ ] Wrote necessary unit or integration tests.