feat: support custom keys for custom Open AI models

[sdirix]

What it does

The configuration for custom OpenAI models now allows specifying a unique 'apiKey' for each model, or reusing the global OpenAI API key.

fixes #14288

How to test

• Add a custom Open AI model (does not need to be reachable for testing)
• Debug the "initializeOpenAi" method in the Open AI language models
  • should use no-key if no apiKey is specified
  • should use the specified key in apiKey if it exists
  • should use the global key (preference or environment variable) if apiKey is set to true

Note that during my testing not every nested change of the custom model preference resulted in a preference notification. To be safe you can reload the whole window which will trigger a "force-read" of the preferences. I think this issue is unrelated to the changes here.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[planger]

Thanks, looks good to me! 👍 Works as advertised. I just tested it via debugging through the code, but not with an actual custom OpenAI compatible model though.