Allow user to change LLM as they can in Python

## Description

If you are using Python, it is quite straightforward to switch the LLM even to OpenAI. Instructions are here: https://github.com/harmonydata/harmony#using-openai-or-other-llms-for-vectorisation - however there isn't currently a way to do this from R/ Docker.

## Rationale

Users want to try more modern LLMs including cloud based LLMs. But how to expose from R?

Start with the linked Python/OpenAI instructions and inspect the repository's R and Docker entry points for vectorisation. Define how users select an alternative LLM in both interfaces, and verify that the chosen model can be used as described in the Python workflow.

Written by the indexing model from the issue text.