[C-API] Optional user_data parameter for callbacks

[Macoron]

Pass an extra argument in callbacks, like llmodel_prompt_callback, llmodel_response_callback, etc. It would be really handy to pass void* parameter to callbacks, that can be filled by user custom data.

For example, because I use static callbacks in C# IL2CPP, I need to track called object somehow. I can use that extra optional parameter in callback to return from static context to object context. Here is example from whisper.cpp implementation.