[GNU Mailman]Support for REST Callbacks in Core

After Postorius and Hyperkitty call the API of Mailman Core to manage the data, they need to poll the Core to confirm that the modification has been successfully executed. This blind query mechanism will increase network and server load and be difficult to scale. This theme uses NATS's publish subscribe mechanism to optimize the steps of getting modification result. After the modification is successfully executed in Mailman Core, a message will be posted on the publish-subscribe(pub-sub) topic. Postorius and HyperKitty can obtain information about the modification result by subscribing to the pub-sub topic and then proceed to the other step.