Add OpenAI-compatible API support with configurable endpoints and models

[darkmuck]

Add OpenAI-compatible API support with configurable endpoints and models

Allows users to specify custom API base URLs and model selections through environment variables while maintaining backward compatibility with the official OpenAI API.

[darkmuck]

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[jack-williams]

@darkmuck Sorry I let this sit for so long. Merging. Thank you for contributing.