Existing options keys

Hi @amontoison @tmigot @dpo!

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 when solving with Knitro.

```julia
solver = NLPModelsKnitro.KnitroSolver(nlp; options...)
NLPModelsKnitro.solve!(solver, nlp)
```

Is there a way to get the possible keys?

Thanks!

Start with the KnitroSolver constructor and solve! entry points shown in the issue, then trace how the options... arguments are handled. Compare the wrapper's available option handling with the KNITRO interface and define a way for users to discover valid option keys; done means the supported keys can be retrieved before solving.

Written by the indexing model from the issue text.