Add stubgen autogeneration

[bakpaul]

At first I wanted to make it auto generated in the CMake project, to have a well integrated workflow in the build. I tried in many ways :

1. Tried to do it during the build (see first commit). But because ninja offers no guaranty over the dependency effective termination (see https://cmake.org/cmake/help/latest/command/add_dependencies.html#command:add_dependencies) it doesn't work.
2. Tried to do it through a add_custom_command but it isn't possible because the targets SofaPython3 and Bindings are Interfaces.
3. Also tried to do it during the install process which is a nightmare because the installation order isn't ordered.

So back to the weak solution, a script that needs to be run by hands. I'll really enjoy adding it to Jenkins...

[alxbilger]

I would prefer the script generate_stubs to be written in Python. It would more readable and cross-platform.