Allow providing custom fetch impl

[yujonglee]

Resolves #466

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[yujonglee]

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[yujonglee]

Motivation: If LMStudioClient accept custom fetch, we(Hyprnote) don't have to do fetch call in the webview, therefore no CORS enabling needed.

[ryan-the-crayon]

Thanks for the PR. However, lmstudio.js communicates with LM Studio via WebSocket for better resources management and better interactivity purposes. It appears this PR only replaced the fetch used for server discovery. We do enforce CORS even on our WebSocket connections. Specifically, if CORS is not turned on, we reject connections that have an "origin" header. So your backend will need to be able to proxy WebScoket connections as well.

If your environment can only make use of regular http requests, I would recommend using our Open AI compatible endpoints here: https://lmstudio.ai/docs/developer/openai-compat