feat(`learner`): create configurable `LLMClient`

[Autoparallel]

Feature Description
We should allow for extending the interface for the LLM APIs. We only have the ollama API currently, and this is by no means going to be acceptable as people will want OpenAI, Anthropic, or MLX.

Motivation
Allows for easier extension of the base lib and allows people to use other LLMs. Should be configurable like the Retriever

Implementation Details
If applicable, describe the technical approach to implementing this feature:

• Create suitable configuration for a generic LLM

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I am also not married to the name LLMClient or using Client at all. I think there are likely other better fitting names!