Expose Python code generation via rosidl generate CLI

[hidmic]

Built on top of ros2/rosidl#567. Connected to ros2/rosidl#565.

[hidmic]

This works to a partial extent. Current generator code assumes the namespace structure in IDL files replicates their typical layout in the filesystem i.e. in a given ROS package within a subdirectory, to the point it uses either interchangeably. If the above convention is not respected, this code generator falls apart. I fear it's not the only code generator doing this.

[ahcorde]

Need rosidl_cli release https://build.ros2.org/job/Rpr__rosidl_python__ubuntu_focal_amd64/14/

[hidmic]

CI up to rosidl_generator_py:

• Linux
• Linux-aarch64
• macOS
• Windows