pybind/mgr: Add ceph_module.pyi to improve type checking

pybind/mgr: Add ceph_module.pyi to improve type checking

ceph_module.pyi is an interface definition of classes that are generated within C++.
Used by mypy to do proper type checking of mgr modules.
Without this file, all classes have undefined base classes.

Now, we're also finding accesses to undefinied class properties.

Secondly, this also improves the development experience when using IDEs.

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>