Add CompletionModel class - wider scope

[maciej-cz]

First of all, thanks for your effort of creating this project @StefanBratanov . It makes OpenAI usage much more convenient.

Now, to the point, the purpose of this change is to replace completion model parameter (currently String) with a value object with predefined values available.

I created 2 versions of this change (the other one is #5). This one is a bit broader:

• CompletionModel is an interface with 2 subclases: ContinuousCompletionModel enum and CompletionModelId
• String value is no longer used for storing model, even internally.

Please suggest, which version you prefer (if any).

In case of positive feedback, I can spread this change to other APIs' models.

[StefanBratanov]

As discussed, will close this in favour of #5