Add Paligemma support

[abetlen]

Weights and example code: https://huggingface.co/abetlen/paligemma-3b-mix-224-gguf

• the chat handler here only supports single image / single turn conversations, just did this to simplify testing but will change before merging
• you must set n_ubatch to be sufficiently large to process both the image and text input embeddings together.