Consider adding MessageId to ChatMessage and ChatResponseUpdate

[stephentoub]

As tools and agents become more prevalent, it's more likely to have multiple messages that are part of a single overall operation, which is represented as a ChatResponse. We have a ResponseId on ChatResponse, but we lack a unique identifer on each invidual ChatMessage that makes up the ChatResponse. Similarly, ChatResponseUpdate has a ResponseId on it, but it doesn't have a MessageId. We end up using the ChatResponseUpdate.ResponseId logically as a message ID to differentiate where one message ends and another begins in a stream, but this could be made cleaner by making the grouping of updates into messages more explicit by having a MessageId on each ChatResponseUpdate in addition to the ResponseId.