Existing options keys

Hi @tmigot @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 when solving with Ipopt.

```julia
solver = NLPModelsIpopt.IpoptSolver(nlp)
NLPModelsIpopt.solve!(solver, nlp; options...)
```

Is there a way to get the possible keys? I am aware of this page but the list is long: [Ipopt options](https://coin-or.github.io/Ipopt/OPTIONS.html).

Thanks!