Consider using a builder class for creating Messages

[markpollack]

See #427 and related issue #1592

[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.

[ThomasVitale]

UserMessage and SystemMessage have builders now. We could use one for AssistantMessage as well.