Custom plugin fields for users

### Describe the problem to be solved

The aim is to allow to add custom fields / private or not , so that we could have more specific public info about the user. The user's ciustom field could be edited by the user and / or by admin. I will look in the code when I have some times but if anyone that knows well the code could guide me a little this would be nice.

### Describe the solution you would like

_No response_

No files, tests, or entry points are named, so first map where user fields are represented and edited. Clarify the private/public behavior and whether users, administrators, or both may edit fields before implementation. Done means custom user fields support the agreed visibility and editing rules.

Written by the indexing model from the issue text.