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/cluster_message.go

@@ -3,11 +3,6 @@
 
 package model
 
-import (
-	"encoding/json"
-	"io"
-)
-
 type ClusterEvent string
 
 const (
@@ -62,17 +57,6 @@ type ClusterMessage struct {
 	Event            ClusterEvent      `json:"event"`
 	SendType         string            `json:"-"`
 	WaitForAllToSend bool              `json:"-"`
-	Data             string            `json:"data,omitempty"`
+	Data             []byte            `json:"data,omitempty"`
 	Props            map[string]string `json:"props,omitempty"`
 }
-
-func (o *ClusterMessage) ToJson() string {
-	b, _ := json.Marshal(o)
-	return string(b)
-}
-
-func ClusterMessageFromJson(data io.Reader) *ClusterMessage {
-	var o *ClusterMessage
-	json.NewDecoder(data).Decode(&o)
-	return o
-}