Existing options keys

Hi @amontoison @abelsiqueira and others!

I am working on [control-toolbox](https://github.com/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!