Option to lock full name when using external auth sources

[thezzisu]

Feature Description

In certain scenarios (such as within schools), users registered through external authentication need to maintain their real names.

Since names may contain Unicode characters (like a Chinese name), a good practice is to set DEFAULT_SHOW_FULL_NAME=true and set the user's full name to their real name. In this situation, it's necessary to ensure that external users cannot modify their own full name.

An option like EXTERNAL_USER_LOCK_FULL_NAME would be great to fit this need.

Screenshots

[thezzisu]

I am currently writing a PR for this.
Also, a working example is implemented in a fork, but I think it would be better to have this functionality built-in.