Add ping method to auth engines to determine if they are reachable

data/users/externaljwt.py

@@ -37,6 +37,12 @@ class ExternalJWTAuthN(FederatedUsers):
     with open(public_key_path) as public_key_file:
       self.public_key = public_key_file.read()
 
+  def ping(self):
+    result = self.client.get(self.getuser_url, timeout=2)
+    if result.status_code // 100 != 4:
+      return (False, result.text or 'Could not reach JWT authn endpoint')
+
+    return (True, None)
 
   def get_user(self, username_or_email):
     if self.getuser_url is None: