Spring-ai OpenAI ChatClient tools support

Add support for ChatClient function calling. - Add generic (model) ToolFunctionCallback and AbstractToolFunctionCallback abstractions for defining and registering function callbacks with the ChatClient. - OpenAiChatClient Function calling implementation. - Unit/IT tests for OpenAiApi and OpenAiChatClient function calling. - Add function calling docs.

Comment From: markpollack

removed the default methods in the chat option interface to make tests pass.

Merged as ffd8222c209967552ea32a265837a48e72076b13

Can review the small comments in this PR afterwards