[md][ff-pm] Add freeflow porous medium coupling managers and test
Part of merging !2201 in smaller parts.
Improvement in comparison to old convergence test:
- Add stress tensor for analytical solutions
- add tests for dirichlet, neumann and mixed dirichlet-neumann boundaries in both domains
- test all implemented analytical solutions
- Coupling manager: Make interface smaller (private) whereever possible
- Fix updateCouplingContext