New `LOCAL_LLM_MODEL` env var in `ai-service` to select a given local LLM model

[mathieu-benoit]

When having multiple local LLM models, it would be great to have an option to have a new LOCAL_LLM_MODEL env var in ai-service to select a given one.

Today, the first one in the list is selected: https://github.com/Azure-Samples/aks-store-demo/blob/main/src/ai-service/routers/description_generator.py#L64.

Current behavior could be kept if this new LOCAL_LLM_MODEL is not supplied or empty.

[pauldotyu]

Thanks for the suggestion @mathieu-benoit. I'll look to implement this in the next release.