Rename host_id map getter, 'get_endpoint_to_host_id_map_for_reading' to 'get_endpoint_to_host_id_map_'
Also modify the getter to return information regarding joining nodes as well.
This getter will later be used for retrieving the nodes in nodetool status, hence it needs to show all nodes,
including joining ones.
The function name suffix `_for_reading` suggests that the function was used
in some other places in the past, and indeed if we need endpoints
"for reading" then we cannot show joining endpoints. But it was confirmed
that this function is currently only used by "/storage_service/host_id" endpoint,
hence it can be modified as required.
Fixes: scylladb/scylladb#17857
72f3c95a63