Add an extension to provide an NLPModels

[amontoison]

We should do an extension to add a method nomad(nlp::AbstractNLPModel).

[salomonl]

I would personally prefer to keep a minimal number of dependencies for this package. In my opinion, if one wants to add an extension, it could be done in another library. However, it could be considered if we integrate it into JuMP (the number of dependencies will increase; so this recommendation is not set in stone).

[amontoison]

@salomonl I don't plan to add a dependency.
Do you know what is a package extension?
It's available since Julia 1.9: https://pkgdocs.julialang.org/v1/creating-packages/#Conditional-loading-of-code-in-packages-(Extensions)

[amontoison]

@salomonl
An example with PRIMA.jl: https://github.com/libprima/PRIMA.jl/blob/main/ext/PRIMANLPModelsExt.jl