feat: add custom openai host endpoint

[AnthonyRonning]

This is a first start at adding a custom openai-compatible endpoint. I have this working with my LiteLLM proxy.

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Some open issues I've caught so far:

1. Custom models do work but the "available models" could use work. One option here is to make this list dynamic from the /models endpoint that I believe most openai-compatible APIs should have.

2. Consequently, the "browse models" link doesn't do anything

3. Could also add the ability to pass in the model(s) during configuring it.

4. Could also add the ability to provide multiple custom openai-compatible models but would increase the scope a lot, I would imagine. Could be done in a follow up if there's enough demand.

I'm open to any opinions here for how to handle these issues or postpone for now.

Closes: #885

[yingjiehe-xyz]

can we directly use the OPEN AI provider? you can set the host and api key to it

[AnthonyRonning]

can we directly use the OPEN AI provider? you can set the host and api key to it

Definitely possible and could be a lot more simple. Though some of the open questions about handling the models would remain. Still functional without needing to answer those questions yet, it will just require a few manual steps that might not be known to the user right off the bat.