[Evals API][9/n] SimpleQA evals

[yanxi0830]

--continuation of https://github.com/meta-llama/llama-stack/pull/352

TL;DR

1. Implement OpenAI's SimpleQA's Benchmark as ScoringFn (reference)

[RFC]

• Option 1: SimpleQAScoringFn: Move each benchmark eval into separate scoring function with it's own context.
• Option 2 (current): Single LLMAsJudgeScoring for SimpleQA.

Test

• Test client app in: https://github.com/meta-llama/llama-stack-apps/pull/105