[FEA] Add a way to retrieve the solver type from the solution object

[nguidotti]

In the current API, there is no way to retrieve which solver was used for solving the LP from the solution object (optimization_problem_solution_t in C++) when using concurrent mode. To enable this, we need to change the API from C++, C and Python.

We also should update the solve_by_pdlp flag since a LP can be solved with barrier, dual simplex and pdlp when running in concurrent mode.