Remove OpenAI SDK dependency and extend it from Drupal AI core module

It looks like we also need this issue for https://www.drupal.org/project/ai_provider_amazeeio → v1.2

Taking a look at the MR

It appears to work, at least if I use amazee.ai endpoint as the LiteLLM and use a key from there, then I can see LiteLLM as an available provider

However, I don't see models but I think that's an issue on the amazee.ai provider (api) side and not the LiteLLM module (as I can reproduce the issue for the amazee.ai provider module).

We may need to bump this to 1.2.x branch like for ai_provider_openai.

Tagging to 1.2.x and also fixing some issues that were still missing:

1. Typo
2. Remove OpenAI Dependency completely
3. Reinstated the custom model loader - its different then the normal OpenAI client based one

Tested locally using a key for amazee.ai as the LiteLLM provider.

Works as expected, able to select models and run chat generation.

Merged to 1.2.x

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