feat: Support custom headers and pre-request auth hooks for OpenAI-compatible providers

[qznc]

Fixes #119

[qznc]

Unit tests are passing, but I haven't done any real testing yet.

[qznc]

It does work in practice and well enough for my purposes, so it is ready from my point of view.

[chatgpt-codex-connector]

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[danielytics]

I'm interested in seeing this merged, but I'm wondering if a simpler (ie lower surface area to review) approach might be to selectively add this feature only to backends that are likely to be used with other providers (eg OpenAI, which most providers are compatible with and perhaps Anthropic which at least MiniMax has a compatible API for, although they do support OpenAI too).

The reason being that the normal backends don't require it, only if you piggyback a different provider on one. Another approach is to make new backends OpenAICompat, although that would be more code to review.