[Feature Request]: Allow default_headers to be passed to OpenAI client

[albertkklam]

Do you need to file an issue?

• [X] I have searched the existing issues and this feature is not already filed.
• [X] My model is hosted on OpenAI or Azure. If not, please look at the "model providers" issue and don't file a new one here.
• [X] I believe this is a legitimate feature request, not just a question. If this is a question, please use the Discussions area.

Is your feature request related to a problem? Please describe.

The OpenAI client allows default_headers to be passed to it when instantiating the client, but this is not supported currently in GraphRAG.

Describe the solution you'd like

Users can save their default headers in the settings.yaml or config.yaml file when initializing GraphRAG, and these values can be passed to the default_headers argument when instantiating the OpenAI client.

Additional context

No response