feat: add OpenAI configuration options to Settings and update LLM client setup

[paul-paliychuk]

:rocket:	This description was created by Ellipsis for commit 616234055123104d4142d9411f5f44e809d7d5f8

feat: add OpenAI configuration options to Settings and update LLM client setup

Summary:

Add OpenAI configuration options to Settings and update get_graphiti for LLM client setup.

Key points:

• Add OpenAI configuration options to Settings in config.py.
• Add openai_base_url and model_name fields.
• Update get_graphiti in zep_graphiti.py.
• Configure client.llm_client with openai_base_url, openai_api_key, and model_name.

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