Support codegen in editable install

[vineetbansal]

Right now python setup.py develop or pip install -e . do not pull in the codegen_src folder (generated during extension build time, which should be theoretically be identical in editable/normal install modes). This won't matter to 99% of the users, but it would be nice to investigate why this is not working and fix it, or document clearly that codegen is not supported when installing osqp-python in editable mode.

[vineetbansal]

This may not be an issue for the 1.0 release (need to confirm this) that uses the skbuild backend, but is not yet fixed (as of 05/22/24) for the 0.6.x releases.