Spring-ai Add support for getting usage tokens for openaiAPI

fix #814

Comment From: tzolov

@YanjiuHe I believe the 20e4b5609a35c204b0e93ac6fcd38ae821a86d1d resolves https://github.com/spring-projects/spring-ai/issues/814 ?

Comment From: markpollack

I'm going to be reviewing the classes for ChatResponse and metadata, I think it was a mistake to inherit from HashMap, instead a hashmap should be used as a field to capture any non-standard metadata.

These changes only move the storage of the objects into the hashmap, the functionality to get usage tokens is already available. Am I missing something?