Add new Responses API endpoint

[simonx1]

Add support for OpenAI Responses API

This PR adds support for OpenAI's Responses API, which is their most advanced interface for generating model responses. The API supports text and image inputs with text outputs, and allows for stateful interactions using previous response IDs.

Features added:

• Basic client method for making requests to /responses endpoint
• Support for follow-up messages (similar to the former threads functionality in the Assistant API)
• Tool calling functionality
• Streaming capability with both proc and callable object support
• Comprehensive test coverage with VCR cassettes

Documentation:

• Added Responses API section to the README with examples for:
  • Basic usage
  • Follow-up messages
  • Tool calls
  • Streaming

This implementation follows the same patterns used throughout the rest of the gem for consistency.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?

[alexrudall]

Legend @simonx1. Will aim to get this released tomorrow

[MilanVilov]

Great job!

[jeduardo824]

❤️

[SerhiiBorozenets]

Cool, thanks!