Add Grok as a provider

[charvimehradu]

Pull Request for Issue #11

Summary of Changes

Added GrokProvider class with core functionality:

• Supports the /v1/chat/completions endpoint for chat completions using Grok models
• Supports the /v1/chat/completions endpoint for vision capability
• Supports the /v1/images/generations endpoint for image generation (grok-2-image, grok-2-image-latest)
• Supports the /v1/embeddings endpoint for text embeddings (v1 model)
• Supports the /v1/models endpoint for listing available models

Testing Instructions

Added unit tests for each capability in tests/GrokProvider/

Documentation Changes Required

• Update the framework documentation to include grok as an available provider
• Document the XAI_API_KEY environment variable requirement
• Add usage examples for chat, vision, image generation, and embeddings with Grok