Add pw2qmcpack plugin to CMake

This is the converter to transform QE orbitals/density and other data into a QMCPACK readable file. We used to live downstream with patches but it was so painful. Now it can be integrated as a plugin and connected via CMake as git submodule.

QMCPACK users can enjoy QE develop via -DQE_ENABLE_PLUGINS=pw2qmcpack option.