[Evals API][8/n] AnswerParsingScoringFn for MMLU

[yanxi0830]

--continuation of https://github.com/meta-llama/llama-stack/pull/333

TL;DR

1. Introduce a registerable AnswerParsingScoringFn with AnswerParsingScoringContext for registering scoring functions with context
2. Remove parameters field (context alone is sufficient for things related to scoring context)
3. AnswerParsingScoringFn is able to run all benchmarks with multiple choice tasks (see apps PR).

Test

PROVIDER_ID=test-meta PROVIDER_CONFIG=llama_stack/providers/tests/scoring/provider_config_example.yaml pytest -s llama_stack/providers/tests/scoring/test_scoring.py --tb=short --disable-warnings

App

• See App: https://github.com/meta-llama/llama-stack-apps/pull/105