Improved support for images & assistants

New Features:

1. Added support for image generation endpoint

   • New method: image_generation(params)

2. Added new endpoints for Assistants API:

   • assistants(): Get list of assistants
   • threads(): Get list of threads
   • thread_messages(thread_id): Get messages for a specific thread
   • delete_thread(thread_id): Delete a specific thread

3. Added new endpoints for managing files:

   • files(): Get list of files
   • file(file_id): Get details of a specific file
   • delete_file(file_id): Delete a specific file

Improvements:

1. Updated content format to support image_url input for GPT Vision

   • Added support for image_url type in content format

2. Made authorization optional

   • Allows usage with local models that don't require API key