The `api_url` setting is one that most providers already support and can
be changed via the `settings.json`. We're adding the ability to change
it via the UI for OpenAI specifically so it can be more easily connected
to v0.

Release Notes:

- agent: Added ability to change the API base URL for OpenAI via the UI

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Co-authored-by: Bennet Bo Fenner <53836821+bennetbo@users.noreply.github.com>