Support gpustack

[graysonchen]

Add GPUStack Provider Support

Description

This PR adds support for the GPUStack API as a new provider in RubyLLM. GPUStack is integrated as an OpenAI-compatible provider with specific adaptations for its API requirements.

Changes

• Added GPUStack provider module with necessary configuration settings
• Implemented model listing capability with appropriate mappings for GPUStack's model format
• Added provider-specific chat functionality
• Registered GPUStack as an official provider in the RubyLLM ecosystem
• Updated configuration to include GPUStack API base URL and API key settings

How to use

Configure your API credentials in your application:

RubyLLM.configure do |config|
  config.gpustack_api_base = 'https://your-gpustack-endpoint/v1'
  config.gpustack_api_key = 'your-gpustack-api-key'
  config.default_model = "your-gpustack-model-name"
end

Then use it like any other provider:

RubyLLM.models.refresh!

chat = RubyLLM.chat
chat.ask("What can you do?")

Testing

The implementation has been tested with the GPUStack API to ensure compatibility and proper functionality.