Specify type for scalar "ID" explicitly (with converter)

[WipeAir]

There should be the possibility to set an explicit type for the "ID" scalar. This functionality should also come with a convert, as the ID is typically given as a String.

This may be implemented like this:

• Two new properties within the CodegenOptions. (Reference to the ID type and the converter implementation)
• Special handling for the ID scalar within the TypeMapper.
• Generate a converter interface.