Add a solver_specific entry for RegularizedExecutionStats

[MaxenceGollier]

It would be nice to let the user acess the current regularization parameter \sigma if he wants to do something with it in a callback (it would be useful for me in my penalty algorithm at least).
I suggest doing this by adding the :regularization_parameter entry as a solver_specific of RegularizedExecutionStats types.

[dpo]

Agreed. To be complete, the user should also have access to the trust-region radius, then.