Allow custom solvers

[bagibence]

Currently solvers are created based on the algorithm's name, which for most users is fine and allows checking for compatibility with the regularizer.

However, it could be allowed for user-defined solvers to be passed to BaseRegressor instead of a string.
As long as a solver implements the AbstractSolver interface, it should be compatible with NeMoS.

In this case the compatibility check with the regularizer could be disabled or rewritten, which should be fine since only advanced users would need this option.

[bagibence]

Adding a protocol that defines the interface and checks the existence of the required methods could be useful for checking solvers passed by the user. It doesn't check types though.