Spin LLM support in Python

[mikkelhegn]

Following the release of the LLM apis in Spin, it would be great to also have support for those apis in the Python SDK. Ref: https://github.com/fermyon/spin/blob/main/wit/preview2/llm.wit

[benwilde]

Thanks @mikkelhegn!

[endocrimes]

partial support (inferencing sans embedding) is now in #51

[karthik2804]

This is now completed.