Spring-ai Consider using a builder class for creating Messages

See https://github.com/spring-projects/spring-ai/pull/427 and related issue https://github.com/spring-projects/spring-ai/issues/1592

Comment From: markpollack

Looking at the classes, a builder (inner class) for UserMessage looks to be useful. Perhaps also for AssistantMessage. The other Message classes have very few constructor arguments, so the only reason there would be for consistency.