Get the names and IDs of the extra roles directly from the external app

For now I think this is fine, but in the long run we probably should find a way to get the names and IDs of the extra roles directly from the external app

_Originally posted by @marioncottard in https://github.com/baobabsoluciones/cornflow/pull/648#discussion_r2129212197_

The issue names no files, tests, or entry points. Start by locating the existing handling of extra roles and the integration with the external app; determine where role names and IDs currently come from. Done would mean the required names and IDs are obtained directly from that external app, with the behavior covered by appropriate tests.

Written by the indexing model from the issue text.