Automatically just `send` streamed messages if in group-scope

We currently do some level of accumulation for messages anyway.
For `group` scope, we have to tell the user to do a check:
`if activity.conversation.isGroup`, then `send`
Otherwise, you can `stream`.

This leads to poor devex. We can abstract this away for them:
In group scopes, we accumulate all the chunks for them. Then once streaming is done, we can just `send` it for them.