Make WebhookConfig exclusive on url (#7835)

Config objects need a an explicit unique property set, so instead of
using a synthetic random value, make it unique on the URL. This means
adding more events to the same URL is updating the event set rather than
having a separate config for that event.

edb/lib/ext/auth.edgeql

@@ -377,6 +377,8 @@ CREATE EXTENSION PACKAGE auth VERSION '1.0' {
         create required property url: std::str {
             create annotation std::description :=
                 "The url to send webhooks to.";
+
+            create constraint exclusive;
         };
 
         create required multi property events: ext::auth::WebhookEvent {