Gitlab::LDAP::Person uses LDAP attributes configuration

What does this MR do?

We allow users to configure LDAP attribute preferences. For example, email can be configured to use mail, email and userPrincipalName, falling through to the next until a value is found. Prior to this change, Gitlab::LDAP::Person did not honor this configuration. Now, the class will honor name and mail configuration. It does not handle username, or fallback to first_name + last_name in the absence of name.

Closes #26290 (closed)

cc/ @MrChrisW