With #20695 we removed local transport and there is just TransportAddress now. The
UnicastHostsProvider currently returns DiscoveryNode instances, where, during pinging, we're
actually only making use of the TransportAddress to establish a first connection to the possible new
node. To simplify the interface, we can just return a list of transport addresses instead, which
means that it's not necessary anymore to create fake node objects in each plugin just to return the
address information.