Create shared user mapping in PostgreSQL foreign data wrapper server
The only way to share a user mapping is if you define it for PUBLIC, but then it applies to all users. Groups don't work in that context, probably because it would cause ambiguity if several user mappings apply to a single user through inheritance.
If you don't want to go with a user mapping per user, you could opt to create one for PUBLIC but limit access to the foreign tables to role_name through permissions.
I know that this is not a great solution either, because it will allow all users to see the user mapping and the password in there (if there is one). But at least it is easy to GRANT SELECT ON ALL TABLES IN SCHEMA ... TO role_name.