Broadcast operators and broadcast events

[jspenger]

There is currently no "broadcast" operator or event type in Portals. Having a broadcast operator would enable sending a message to all instances of a task. This can be used to set some state on all instances of an operator. Other use cases are to send a Portal request to all instances of a task. I think it is fairly clear that this operation can be very beneficial. However, it is not clear what the semantics of this operation should be, and what the operator should look like, etc. This will need further deliberation.