Add xAI as an OpenAI compatible provider

[dkotter]

This PR adds in a new AI provider, xAI, using the existing OpenAI compatible classes. xAI claims to have full compatibility with the OpenAI REST API, making it fairly straightforward to add.

I've added support for both text generation and image generation, as well as support for both text and image input modalities.

This can be quickly tested using our cli.php command:

XAI_API_KEY=xai-KEY php cli.php 'Hello' --providerId=xai
XAI_API_KEY=xai-KEY php cli.php 'Hello' --providerId=xai --modelId=grok-3-mini

Note

I know there's an existing roadmap and adding new providers is currently not on that roadmap. I also know there was a discussion on which providers to initially support, with xAI not being on that list.

Happy to close this out if we don't want to consider this right now or we can also look at supporting this as a stand-alone, 3rd party thing under the 10up/Fueled repo if we don't want to support xAI directly in this package

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