Support OpenAI-style endpoints

[mcharytoniuk]

See:

https://github.com/distantmagic/paddler/discussions/16#discussioncomment-10401611 https://github.com/distantmagic/paddler/issues/6#issuecomment-2297674409

The lack of slot support can be worked around at the agent/observer level so that information does not have to be obtained just through llama.cpp.