Implement adjacency_matrix and expression in _FunctionStorage

[blegat]

EAGO is accessing the objective and constraints fields of ReverseAD.NLPEvaluator which is ok since it's not starting with a _.
But then it is accessing internal fields of _FunctionStorage and _SubexpressionStorage which it shouldn't since these start with an _.
Actually, all they need are the nodes and const_values which are part of MOI.Nonlinear.Expression and the adjacency matrix so by implementing adjacency_matrix and adding a expression function that it implements, it allows EAGO to stop using internal fields and stick to the public API.

See the corresponding EAGO PR here : PSORLab/EAGO.jl#147

[odow]

I'd prefer we just closed #2747

[odow]

Close this as well?

[blegat]

Yes, let's close