refactor: adding custom headers to KeycloakOpenID

Adding `custom_headers` to KeycloakOpenID instantiation.

src/keycloak/openid_connection.py

@@ -300,6 +300,7 @@ class KeycloakOpenIDConnection(ConnectionManager):
                 verify=self.verify,
                 client_secret_key=self.client_secret_key,
                 timeout=self.timeout,
+                custom_headers=self.custom_headers,
             )
 
         return self._keycloak_openid