Make python geometry msgs typecastable into C++ types automatically

[adityapande-1995]

Feature request

Feature description

For instance, When writing pybind11 bindings for functions with geometry::msgs::Quaternion as an argument, geometry_msgs.msg.Quaternion cannot be supplied on the python side, as the two types cannot be typecasted readily into each other.

It would be nice to have a smoother way to convert between these two, instead of writing a custom typecaster every time.

Implementation considerations

We could write a custom typecaster for these somewhere in ros2 repos, or make the python type itself derive from C++ type somehow.

cc @sloretz