Optimize precomputing of websocket event (#22465)

fmt.Sprintf was badly optimized and in cases of heavy traffic,
it was in the hot path and caused a lot of allocations.

We sacrifice readability for performance here.

```
    10:46:40-~/mattermost/mattermost-server/model] benchstat base.txt new.txt
    goos: linux
    goarch: amd64
    pkg: github.com/mattermost/mattermost-server/v6/model
    cpu: Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz
                 │  base.txt   │               new.txt               │
                 │   sec/op    │   sec/op     vs base                │
    EncodeJSON-8   2.061µ ± 1%   1.588µ ± 2%  -22.93% (p=0.000 n=10)

                 │  base.txt  │              new.txt               │
                 │    B/op    │    B/op     vs base                │
    EncodeJSON-8   545.0 ± 0%   472.0 ± 0%  -13.39% (p=0.000 n=10)

                 │  base.txt  │              new.txt               │
                 │ allocs/op  │ allocs/op   vs base                │
    EncodeJSON-8   6.000 ± 0%   3.000 ± 0%  -50.00% (p=0.000 n=10)
```

```release-note
Writes to websocket now take 13% less memory
and also 22% faster per message.
```

model/websocket_message_test.go

@@ -5,6 +5,8 @@ package model
 
 import (
 	"bytes"
+	"encoding/json"
+	"io"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -235,3 +237,18 @@ func TestWebSocketEventDeepCopy(t *testing.T) {
 	})
 	require.NotEqual(t, ev.data, evCopy.data)
 }
+
+var err error
+
+func BenchmarkEncodeJSON(b *testing.B) {
+	message := NewWebSocketEvent(WebsocketEventUserAdded, "", "channelID", "", nil, "")
+	message.Add("user_id", "userID")
+	message.Add("team_id", "teamID")
+
+	ev := message.PrecomputeJSON()
+
+	enc := json.NewEncoder(io.Discard)
+	for i := 0; i < b.N; i++ {
+		err = ev.Encode(enc)
+	}
+}