Create cmake functions for triggering the shadergen tool

We need infrastructure to enable the shadergen tool to be used in a project. This task is about researching what's needed and implementing it. I think QtShaderTools already has something similar implemented already, which might be a good inspiration. We also have a simple qmake variant for testing purposes here: https://codereview.qt-project.org/c/qt/qtquick3d/+/309335