[Applications/PL] Add initial stress and friction to embedded embedded anchor source term / preprocessing tooling

This Merge request adds initial stress and a simple friction model to the embedded point-to-point anchor elements. The added weak plane models required for the slope stability tests will be tested more thoroughly by MR !5382

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?