client/authority-discovery: Terminate when network does (#5562)

* client/authority-discovery: Add test ensure termination on network termi

* client/authority-discovery: Terminate when network does

When the dht event stream returns Poll::Ready(None) it is likely due to
the network terminating. When the network terminates due to the node
itself shutting down or due to a fatal error, there is no purpose in
continuing to run the authority discovery module.

* client/authority-discovery/src/lib: Apply suggestions

Co-Authored-By: André Silva <123550+andresilva@users.noreply.github.com>

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>