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
```

app/plugin_api_tests/manual.test_http_upgrade_websocket_plugin/main.go

@@ -34,7 +34,7 @@ func (p *Plugin) ServeHTTP(_ *plugin.Context, w http.ResponseWriter, r *http.Req
 		}
 		req := model.WebSocketRequestFromJson(bytes.NewReader(msg))
 		resp := model.NewWebSocketResponse("OK", req.Seq, map[string]interface{}{"action": req.Action, "value": req.Data["value"]})
-		if err = ws.WriteMessage(mt, []byte(resp.ToJson())); err != nil {
+		if err = ws.WriteMessage(mt, resp.ToJson()); err != nil {
 			break
 		}
 	}