Allow smooth box constrained problems

With https://github.com/JuliaSmoothOptimizers/ShiftedProximalOperators.jl/pull/156, it should now be possible to use our solvers for unregularized problems.

Users should be able to call the solvers with regular `AbstractNLPModels`, for example

```
nlp = CUTEstModel(...)
solver = R2NSolver(nlp) # Automatically creates a reg_nlp = RegularizedNLPModel(nlp, NullRegularizer())
stats = RegularizedExecutionStats(nlp)
solve!(solver, nlp, stats)
```