MM-25516: Changed to byte slice instead of string for cluster messages (#17998)

* MM-25516: Changed to byte slice instead of string for cluster messages

https://mattermost.atlassian.net/browse/MM-25116

Testing:
Manually tested.
Load-tested with Cluster Controller.

I looked into changing the serialization method to use msgpack,
but the ClusterMessage struct was mainly used for only 3 fields
which didn't lead to much of a CPU time improvement, whereas
actually led to more allocations using msgpack. Hence, I chose
to remain with JSON.

```
name              old time/op    new time/op    delta
ClusterMarshal-8    3.51µs ± 1%    3.10µs ± 2%  -11.59%  (p=0.000 n=9+10)

name              old alloc/op   new alloc/op   delta
ClusterMarshal-8      776B ± 0%     1000B ± 0%  +28.87%  (p=0.000 n=10+10)

name              old allocs/op  new allocs/op  delta
ClusterMarshal-8      12.0 ± 0%      13.0 ± 0%   +8.33%  (p=0.000 n=10+10)
```

```release-note
Changed the field type of Data in model.ClusterMessage to []byte from string.
```

* Trigger CI
```release-note
NONE
```

model/websocket_message.go

@@ -77,7 +77,7 @@ const (
 )
 
 type WebSocketMessage interface {
-	ToJson() string
+	ToJson() []byte
 	IsValid() bool
 	EventType() string
 }
@@ -199,9 +199,9 @@ func (ev *WebSocketEvent) EventType() string {
 	return ev.event
 }
 
-func (ev *WebSocketEvent) ToJson() string {
+func (ev *WebSocketEvent) ToJson() []byte {
 	if ev.precomputedJSON != nil {
-		return fmt.Sprintf(`{"event": %s, "data": %s, "broadcast": %s, "seq": %d}`, ev.precomputedJSON.Event, ev.precomputedJSON.Data, ev.precomputedJSON.Broadcast, ev.GetSequence())
+		return []byte(fmt.Sprintf(`{"event": %s, "data": %s, "broadcast": %s, "seq": %d}`, ev.precomputedJSON.Event, ev.precomputedJSON.Data, ev.precomputedJSON.Broadcast, ev.GetSequence()))
 	}
 	b, _ := json.Marshal(webSocketEventJSON{
 		ev.event,
@@ -209,7 +209,7 @@ func (ev *WebSocketEvent) ToJson() string {
 		ev.broadcast,
 		ev.sequence,
 	})
-	return string(b)
+	return b
 }
 
 // Encode encodes the event to the given encoder.
@@ -280,9 +280,9 @@ func (m *WebSocketResponse) EventType() string {
 	return WebsocketEventResponse
 }
 
-func (m *WebSocketResponse) ToJson() string {
+func (m *WebSocketResponse) ToJson() []byte {
 	b, _ := json.Marshal(m)
-	return string(b)
+	return b
 }
 
 func WebSocketResponseFromJson(data io.Reader) *WebSocketResponse {