This short series fixes Alternator's "/localnodes" request to allow a node's external IP address - configured with `broadcast_rpc_address` - to be listed instead of its usual, internal, IP address.
The first patch fixes a bug in gossiper::get_rpc_address(), which the second patch needs to implement the feature. The second patch also contains regression tests.
Fixes#18711.
Closesscylladb/scylladb#18828
* github.com:scylladb/scylladb:
alternator: fix "/localnodes" to use broadcast_rpc_address
gossiper: fix get_rpc_address() for this node