The actual entity target of the stream is held in this component which does
not have any generic parameters. This means it is possible to lookup the
targets of the streams coming out of the node without knowing the concrete
type of the streams. This is crucial for being able to redirect the stream
targets.
Used by ServiceDiscovery to filter services
based on what streams they provide. If a stream is required, you should wrap
the stream type in Require. If a stream is optional, then wrap it in
Option.
The StreamPack trait defines the interface for a pack of streams. Each
Provider can provide zero, one, or more streams of data
that may be sent out while it’s running. The StreamPack allows those
streams to be packed together as one generic argument.