Add an additional column to the user-table to allow user identification

Is your feature request related to a problem? Please describe.
I import users from csv-file, using a script. The Username is generated out of firstname and lastname. Using the - in the csv-file also excisting -unique-id is not user-friendly because the username is the login-name.
If firstname or lastname changes, there will be generated a new user instead of updating the old one. The only way to identify the excisting user would be by unique-id which can't be added anywhere.
As described here it would be very hard to allow changing username.

Describe the solution you'd like
It would be helpful to identify users by an additional unique-id.
Add a new column "identifier" to the user-table.
The new column "identifier" is shown and used only in the admin-usermanagement/api and is not the unique-id.
The new column "identifier" has not to be set, it is optional.
The values in the new column "identifier" have to be unique and only can be set, viewed and changed by admin/api.

Describe alternatives you've considered
None.

Start with the admin-usermanagement API and the user-table data model described in the issue. Determine how an optional, unique identifier can be stored and exposed only through admin/API operations; done means administrators can set, view, and change it while imports can use it to identify users independently of usernames.