Service: Add support for validation of the planning solution

Is your feature request related to a problem? Please describe.
When validating user input, there are scenarios for which validation of the DTOs alone is not sufficient anymore. Users might submit data with assignments that are already violating a constraint. Detailed validation would lead to code duplication between constraints and validation.

Describe the solution you'd like
Have a second validation phase (probably after enrichment) in the solver service, that validates the actual planning solution and not just the user input. The user input validation could be seen as syntactic validation, while the planning solution validation would be more about semantics.