Add group chat MessageReceive documentation

This fixes the issue in the group chat message processor. The crux of the issue is that we were returning to front end via the callback a JSON marshalled Go structure, and were not making this clear through documentation.

type MessageReceive struct {
	GroupID   *id.ID
	ID        group.MessageID
	Payload   []byte
	SenderID  *id.ID
	Timestamp time.Time
}

This MR creates a bindings layer equivalent and returns a JSON marshalled version of that through the callback, and attempts to make it clear by updating documentation.

bindings/group.go

@@ -410,6 +410,7 @@ type GroupRequest interface {
 }
 
 // GroupChatProcessor manages the handling of received group chat messages.
+// The decryptedMessage field will be a JSON marshalled GroupChatMessage.
 type GroupChatProcessor interface {
 	Process(decryptedMessage, msg, receptionId []byte, ephemeralId,
 		roundId int64, err error)
@@ -422,13 +423,53 @@ type groupChatProcessor struct {
 	bindingsCb GroupChatProcessor
 }
 
+// GroupChatMessage is the bindings layer representation of the
+// [groupChat.MessageReceive].
+//
+// GroupChatMessage Example JSON:
+//  {
+//    "GroupId": "AAAAAAAJlasAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAE",
+//    "SenderId": "AAAAAAAAB8gAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD",
+//    "MessageId": "Zm9ydHkgZml2ZQAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=",
+//    "Payload": "Zm9ydHkgZml2ZQ==",
+//    "Timestamp": 1663009269474079000
+//  }
+type GroupChatMessage struct {
+	// GroupId is the ID of the group that this message was sent on.
+	GroupId []byte
+
+	// SenderId is the ID of the sender of this message.
+	SenderId []byte
+
+	// MessageId is the ID of this group message.
+	MessageId []byte
+
+	// Payload is the content of the message.
+	Payload []byte
+
+	// Timestamp is the time this message was sent on.
+	Timestamp int64
+}
+
+// convertMessageReceive is a helper function which converts a
+// [groupChat.MessageReceive] to the bindings-layer representation GroupChatMessage.
+func convertMessageReceive(decryptedMsg gc.MessageReceive) GroupChatMessage {
+	return GroupChatMessage{
+		GroupId:   decryptedMsg.GroupID.Bytes(),
+		SenderId:  decryptedMsg.SenderID.Bytes(),
+		MessageId: decryptedMsg.ID.Bytes(),
+		Payload:   decryptedMsg.Payload,
+		Timestamp: decryptedMsg.Timestamp.UnixNano(),
+	}
+}
+
 // convertProcessor turns the input of a groupChat.Processor to the
 // binding-layer primitives equivalents within the GroupChatProcessor.Process.
 func convertGroupChatProcessor(decryptedMsg gc.MessageReceive, msg format.Message,
 	receptionID receptionID.EphemeralIdentity, round rounds.Round) (
 	decryptedMessage, message, receptionId []byte, ephemeralId, roundId int64, err error) {
 
-	decryptedMessage, err = json.Marshal(decryptedMsg)
+	decryptedMessage, err = json.Marshal(convertMessageReceive(decryptedMsg))
 	message = msg.Marshal()
 	receptionId = receptionID.Source.Marshal()
 	ephemeralId = receptionID.EphId.Int64()