[Feature] Support DROP exact-match and F1 metrics

[Excelius-Wang]

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Motivation

The OpenAI-derived DROP evaluator currently reports only fuzzy substring accuracy. For example, it treats Denver as fully correct for Denver Broncos, although the standard DROP exact-match score is 0 and F1 is 66.67.

Adding the official metrics makes OpenCompass results comparable with other published DROP evaluations while retaining the existing accuracy output.

Closes #948.

Modification

• add the standard DROP single-span normalization, exact-match, and F1 logic, including numeric normalization and number matching
• take the best EM and F1 over the pipe-delimited alternative reference answers
• preserve the existing accuracy result and add exact_match and f1 aggregates and per-example details
• add focused tests for text and numeric normalization, empty inputs, mismatched numbers, partial overlap, alternative answers, and legacy result compatibility

The simple-evals DROP data stores each pipe-delimited reference as an alternative single-span answer, so this implements the equivalent single-span path of the official metric without adding a SciPy dependency for multi-span alignment.

BC-breaking (Optional)

No. The existing accuracy key and correct detail field are unchanged; the new metrics are additive.

Use cases (Optional)

Users can report standard DROP EM/F1 directly from the existing drop_openai_simple_evals configuration and compare OpenCompass runs with published model results.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects.
• CLA has been signed and all committers have signed the CLA in this PR.