MM-54998: Optimize JSON marshalling in websocket broadcast (#25286)

Marshalling a json.RawMessage is not zero overhead. Instead,
it compacts the raw message which starts to have an overhead
at scale.

https://github.com/golang/go/issues/33422

Since we have full control over the message constructed, we
can simply write the byte slice into the network stream.
This gives considerable performance boost.

```
goos: linux
goarch: amd64
pkg: github.com/mattermost/mattermost/server/public/model
cpu: Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz
             │   old.txt    │              new_2.txt              │
             │    sec/op    │   sec/op     vs base                │
EncodeJSON-8   1640.5n ± 2%   289.6n ± 1%  -82.35% (p=0.000 n=10)

             │  old.txt   │             new_2.txt             │
             │    B/op    │    B/op     vs base               │
EncodeJSON-8   528.0 ± 0%   503.0 ± 0%  -4.73% (p=0.000 n=10)

             │  old.txt   │             new_2.txt              │
             │ allocs/op  │ allocs/op   vs base                │
EncodeJSON-8   5.000 ± 0%   4.000 ± 0%  -20.00% (p=0.000 n=10)
```

P.S. No concerns over changing the model API because we are
still using 0.x

https://mattermost.atlassian.net/browse/MM-54998

```release-note
Improve websocket event marshalling performance
```

server/channels/app/platform/web_conn.go

@@ -463,7 +463,7 @@ func (wc *WebConn) writePump() {
 			var err error
 			if evtOk {
 				evt = evt.SetSequence(wc.Sequence)
-				err = evt.Encode(enc)
+				err = evt.Encode(enc, &buf)
 				wc.Sequence++
 			} else {
 				err = enc.Encode(msg)
@@ -530,7 +530,7 @@ func (wc *WebConn) writeMessage(msg *model.WebSocketEvent) error {
 	// We don't use the encoder from the write pump because it's unwieldy to pass encoders
 	// around, and this is only called during initialization of the webConn.
 	var buf bytes.Buffer
-	err := msg.Encode(json.NewEncoder(&buf))
+	err := msg.Encode(json.NewEncoder(&buf), &buf)
 	if err != nil {
 		mlog.Warn("Error in encoding websocket message", mlog.Err(err))
 		return nil