Add py.typed marker to google-iam-v1 for PEP 561 compliance

The google-iam-v1 package currently lacks a py.typed marker file. Even though the library contains type hints, static analysis tools like mypy fail to recognize them, resulting in the following error for downstream consumers:

error: Skipping analyzing "google.iam.v1": module is installed, but missing library stubs or py.typed marker

Include an empty py.typed file in the root of the google/iam package and ensure it is included in the MANIFEST.in or package_data of the distribution. This will allow mypy to resolve types without requiring ignore_missing_imports = True.