Preparations for a CAD-based arithmetic solver (#4762)

Preparations for a CAD-based arithmetic solver (#4762)

Based on #4679, this PR contains further preparations for a CAD-based arithmetic solver that extends the current NonlinearExtension.
In detail, it provides:

a Constraints class that manages all (polynomial) constraints visible to the cad solver,
a collection of methods used for cad projections,
a VariableOrdering class that provides different variable ordering heuristics tailored to cad,
an extension to the NlModel class, allowing for witness terms,
further conversion methods, in particular between Node and poly::Polynomial, poly::Value and RealAlgebraicNumber.