Add a py.typed file for submodule

[crwilcox]

To be in accordance with PEP 561 we should add a py.typed file to pandas_gbq/py.typed

The file can be empty but does need to be present for some type checkers.

[crwilcox]

Do note, you may need to run mypy, pyright, pytype against our samples before merging. It will ensure that we aren't exposing users to failing type checks.

Ideally we would add this to CI as well, to avoid future breakage.

[tswast]

Blocked by: #325 -- We don't currently publish any type annotations.

[tswast]

Closing as a duplicate of #325