Existing options keys

[ocots]

Hi @amontoison @abelsiqueira and others!

I am working on control-toolbox and I would handle options for the modelers and solvers quite nicely for the user. To do this, I need a way to get all the existing options keys we can pass to ADNLPModel such as :minimize, :backend, `:show_time, etc.

Is there a way to get the keys?

Thanks!