dev: mark the package as typed and strengthen mypy checks

[0x4e3]

The SDK already contains inline type annotations, but the published package does not include a py.typed marker. As a result, type checkers ignore these annotations when the SDK is installed as a dependency.

Proposed changes:

• Add ydb/py.typed to both wheel and source distributions.
• Add the Typing :: Typed package classifier.
• Enable no_implicit_optional and disallow_untyped_decorators in mypy.
• Fix annotations that fail under these stricter checks.
• Preserve decorated function signatures instead of exposing them as generic Callable[..., Any].
• Correct the public scan_query annotations to accept both query strings and ScanQuery objects, with parameters provided as a mapping.
• Exclude generated protobuf modules from SDK-level mypy checks.

I can take this on and prepare a PR within a few days.

I’d also be happy to work on improving the package’s typing from time to time and gradually address other typing issues as they come up.