Add extension to work with multiple LLM

[donhardman]

As discussed, we should implement an extension with ext-php-rs to utilize most of the models for use in conversation RAG and also think about the way this extension will be injected into the binary.

[donhardman]

All done here – https://github.com/manticoresoftware/llm-php-ext

@KlimTodrik Please REVIEW the interface and make sure it's all good and acceptable before I proceed with preparing the executor binary to use it.

[KlimTodrik]

Seems good