Implemented a call interface to LLMModel

[maykcaldas]

This PR implements a call method to LLMModel that input a list of Messages.
Therefore, LLMModel will have a method to process prompts directly (to ensure paper-qa compatibility), run_prompt, and a new method to input a list of Messages, call, which can contain images.

[ludomitch]

LGTM - just one minor docstring comment

[jamesbraza]

I chatted with Mayk in-person, and I understand the current goal is to consolidate everything to this repo with minimal rewrites ("expand phase").

Now that we've mostly completed this, in LLMModel there's many redundant methods:

• achat/achat_iter and acomplete/acomplete_iter giving Chunk variants
• call giving LLMResult
• run_prompt giving LLMResult

When we do the "contract phase", we should standardize on one way. Here's my thoughts on it:

• Standardize on "chat" because its role and message history concepts directly align with Message
• But not name it chat, instead name it call, as llm-client is opinionated towards simplicity
• Remove Chunk in favor of LLMResult