Spring-ai Add Multi-Modality Support for OpenAI

• Implemented a MediaContent abstraction within the OpenAiApi to handle text and image inputs.
• Response message content remains a plain String, ensuring backward compatibility.
• Extended the OpenAiChatClient request creation process to seamlessly map Spring AI Messages with Media content to the low-level OpenAiApi MediaContent types.
• Added integration tests for embedded and URL images, covering both synchronous and streaming calls.
• Updated the OpenAI class diagram to reflect the new media content types provided by the OpenAI API.
• Incorporated a chapter on multi-modality within the openai-chat.adoc documentation.

Comment From: markpollack

merged in 834d2d04879c080da208bdde7cda5aea7c48f585