receipt: include original message sender in receipt events

receipt.go

@@ -63,6 +63,7 @@ func (cli *Client) parseReceipt(node *waBinary.Node) (*events.Receipt, error) {
 		MessageSource: source,
 		Timestamp:     ag.UnixTime("t"),
 		Type:          types.ReceiptType(ag.OptionalString("type")),
+		MessageSender: ag.OptionalJIDOrEmpty("recipient"),
 	}
 	if source.IsGroup && source.Sender.IsEmpty() {
 		participantTags := node.GetChildrenByTag("participants")

types/events/events.go

@@ -385,6 +385,10 @@ type Receipt struct {
 	MessageIDs []types.MessageID
 	Timestamp  time.Time
 	Type       types.ReceiptType
+
+	// When you read the message of another user in a group, this field contains the sender of the message.
+	// For receipts from other users, the message sender is always you.
+	MessageSender types.JID
 }
 
 // ChatPresence is emitted when a chat state update (also known as typing notification) is received.