Backport native Pythia8
This includes a backport of the native Pythia8 generator from @asalzbur with some additional changes:
- Use framework random engines instead of native Pythia8 generator to be consistent with the rest of the framework.
- Adapt to CMake changes.
Depends on !89 (merged) and must be merged afterwards. Fixes ACTSFW-52.
Edited by Andreas Salzburger