Separate announcement sender from libp2p host config

[Masih H. Derkani]

Sender of announcements are a separate construct than publishers. But the current engine implementation implicitly publishes over gossipsub if a libp2p host is present.

Change the engine options such that the sender constructs are clearly separated from publisher and reflect the API exposed by go-libipni.

[Andrew Gillis]

Fixed by #422