Add a moderator_pk for a participant data

common-api/src/api.rs

 #[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct ExchangeMessage {
     pub account_id: AccountId,
+    pub message: Data,
+}
+
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct Data {
+    pub moderator_pk: AccountId,
     pub data: Vec<u8>,
 }