Types are not exported

[gijswobben]

Python type annotations are not exported by default. In order to export types, you have to mark the package with a py.typed file (empty file) or publish types in an external location like typeshed. This PR introduces the py.typed file for the firebase_functions package.

More information on exporting types:

• https://mypy.readthedocs.io/en/stable/installed_packages.html
• https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages
• https://peps.python.org/pep-0561/