Stop advertising Aiven custom authenticator

We decorate standard PasswordAuthenticator with a few platform specific
features, that don't change the externally observable behavior - so it's
absolutely compatible with PasswordAuthenticator.

A few tools and client libraries do not support custom authenticators.
For example: ZDM Proxy. They fail whenever they see anything but
PasswordAuthenticator nevertheless they are in fact completely
compatible.

So we want to stop advertising our custom authenticator and pretend that
it's a standard PasswordAuthenticator.

src/java/org/apache/cassandra/transport/messages/StartupMessage.java

@@ -118,8 +118,12 @@ else if (compression.equals("lz4"))
             clientState.setDriverVersion(options.get(DRIVER_VERSION));
         }
 
-        if (DatabaseDescriptor.getAuthenticator().requireAuthentication())
-            return new AuthenticateMessage(DatabaseDescriptor.getAuthenticator().getClass().getName());
+        if (DatabaseDescriptor.getAuthenticator().requireAuthentication()) {
+            String authenticatorClassName = DatabaseDescriptor.getAuthenticator().getClass().getName();
+            if (authenticatorClassName.equals("io.aiven.cassandra.auth.AivenAuthenticator"))
+                authenticatorClassName = "org.apache.cassandra.auth.PasswordAuthenticator";
+            return new AuthenticateMessage(authenticatorClassName);
+        }
         else
             return new ReadyMessage();
     }