Cleanup support for constrained variables

**Summary**
Current handling for constrained variables is messy / inconsistent. Needs a cleanup for clarity and maintainability.

Not trivial to ask for the sensibility wrt to variable bounds or the dual of variable bounds.

TODO from: https://github.com/jump-dev/DiffOpt.jl/pull/281

Related to: https://github.com/jump-dev/ParametricOptInterface.jl/pull/167
