fix interface handling with generic model

introduce a "Generic" model

This is basically an OpenAI implementation, which then can be reused by
most other providers since they usually implement a somewhat compatible
API. The API URL can be se

introduce a "Generic" model

This is basically an OpenAI implementation, which then can be reused by
most other providers since they usually implement a somewhat compatible
API. The API URL can be set manually so it should be easy to add new
unsupported providers.

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