[FT] add `py.typed` so `lighteval` can work with type checkers

[jamesbraza]

Issue encountered

Seen with lighteval==0.9.2 and mypy==0.15.0:

a.py:1:1: error: Skipping analyzing "lighteval.metrics.utils.metric_utils": module is installed, but missing library stubs or py.typed marker  [import-untyped]
    from lighteval.metrics.utils.metric_utils import SampleLevelMetric
    ^

Solution/Feature

Add a py.typed so type checkers can inspect this package's code.

Possible alternatives

N/a