fix: Fix overly generic type hint on File data source

[keelerm84]

fixes #364

---

Note

Refines Files.new_data_source typing and adds corresponding imports, with no runtime logic changes.

• Types:
  • Narrow Files.new_data_source return type to Optional[Callable[[Config, FeatureStore, Event], UpdateProcessor]].
  • Add required imports: Event, Callable, Config, FeatureStore, UpdateProcessor.

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