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
```

api4/plugin_test.go

@@ -304,11 +304,13 @@ func TestNotifyClusterPluginEvent(t *testing.T) {
 	expectedPluginData := model.PluginEventData{
 		Id: manifest.Id,
 	}
+
+	buf, _ := json.Marshal(expectedPluginData)
 	expectedInstallMessage := &model.ClusterMessage{
 		Event:            model.ClusterEventInstallPlugin,
 		SendType:         model.ClusterSendReliable,
 		WaitForAllToSend: true,
-		Data:             expectedPluginData.ToJson(),
+		Data:             buf,
 	}
 	actualMessages := findClusterMessages(model.ClusterEventInstallPlugin, messages)
 	require.Equal(t, []*model.ClusterMessage{expectedInstallMessage}, actualMessages)
@@ -354,7 +356,7 @@ func TestNotifyClusterPluginEvent(t *testing.T) {
 		Event:            model.ClusterEventRemovePlugin,
 		SendType:         model.ClusterSendReliable,
 		WaitForAllToSend: true,
-		Data:             expectedPluginData.ToJson(),
+		Data:             buf,
 	}
 	actualMessages = findClusterMessages(model.ClusterEventRemovePlugin, messages)
 	require.Equal(t, []*model.ClusterMessage{expectedRemoveMessage}, actualMessages)