Expose OSQP Workspace

[mpowelson]

This is useful so I can do things like this.

    if (solver_.workspace()->info->status_val == -3)
    {
     // Print debugging info
      Eigen::Map<Eigen::VectorXd> primal_certificate(solver_.workspace()->delta_x, num_cnts_, 1);
    }

[S-Dafarra]

LGTM. Actually, I realize that passing a const reference to a unique pointer is not the best in terms of ownership handling, but we are doing the same also for the settings and data structure, so no problem with this PR I guess. If @GiulioRomualdi agrees, for me we can merge.