Add support for pip -e development (editable) mode.
Add some environment vars for configuring the cmake build in
development mode:
- AIDGE_BUILD_GEN=<cmake_backend>: for instance Ninja to use
  the ninja backend which better supports incremental build
- AIDGE_BUILD_MODE=[Debug|Release]: define the build mode,
  defaults to Debug

Note that due to behavior changes depending on the
python/setuptools versions, the portable method to ensure
incremental build on C++ files is to use:

pip install --no-build-isolation -v -e .

Refer to the doc string of setup.py for details.