message: Add domain field to `PubAck`
  See
diff: @@ -439,6 +439,10 @@ pub(crate) struct AccountStats {
  pub(crate) struct PubAck {
  pub stream: String,
  pub seq: u64,
  + // TODO(caspervonb) using String::is_empty as default for String is still unstable.
  + // Use `is_default` once that is no longer gated for strings.
  + #[serde(default, skip_serializing_if = "String::is_empty")]
  + pub domain: String,
  #[serde(default, skip_serializing_if = "is_default")]
  pub duplicate: bool,
  }
  