feat: Jan can load large model with multiple gguf files

[hahuyhoang411]

Problem Jan is only support 1 gguf model file at a time

Success Criteria We can help users to merge gguf files into 1 and load the model for them

Additional context Approach https://www.reddit.com/r/LocalLLaMA/comments/1cf6n18/how_to_use_merge_70b_split_model_ggufpart1of2/