feat: add type checking marker file (and check types in this module, including samples & tests)

[tswast]

I currently get some mypy errors when using this module. Would love to have support for type checking here.

nox > mypy bigframes tests/system tests/unit
error: Skipping analyzing "db_dtypes": module is installed, but missing library stubs or py.typed marker  [import]
note: See [https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports](https://www.google.com/url?q=https://mypy.readthedocs.io/en/stable/running_mypy.html%23missing-imports&sa=D)