@Override
    public String toString() {
        return "UserIdentifier(uniqueId=" + this.uniqueId + ", username=" + this.username + ")";
    }
Creates a {@link UserIdentifier}.

@param uniqueId the uuid of the user
@param username the username of the user, nullable
@return a new identifier
common/src/main/java/me/lucko/luckperms/common/model/UserIdentifier.java