fix: ldap-authorization should be considered external (#8047)

* fix: ldap-authorization should be considered external

When sso auth type was added (1c6b7a9), some code for getting a username
was moved into HttpAuthAuthorizer. LdapAuthorizationAuthorizer uses the
same process to get the username and needs to be updated as well.

* Use common implementation of getExternalUsername

Authorizers get the default implementation by setting AUTH_IS_EXTERNAL.
ADAuthorizationAuthorizer is another candidate.

LibreNMS/Authentication/ADAuthorizationAuthorizer.php

@@ -8,6 +8,7 @@ use LibreNMS\Exceptions\AuthenticationException;
 class ADAuthorizationAuthorizer extends AuthorizerBase
 {
     protected $ldap_connection;
+    protected static $AUTH_IS_EXTERNAL = 1;
 
     public function __construct()
     {