chore: add py.typed to enable typechecking with mypy

[adubovik]

As per PEP-0561 py.typed file must be added to the package so that the type checkers trust the type hints inlined in the Python code.

Pyright works just fine without this file; however, mypy actually relies on it.

The current workaround to make mypy work is to add the following to pyproject.toml:

[[tool.mypy.overrides]]
module = ["aidial_client.*"]
follow_untyped_imports = true