[MM-40485] Enable receiving binary websocket messages (#19128)

* Enable receiving binary websocket messages

* Improve error message

* Prefer anonymous declaration

* Simplify

* Improve test

* Use MessagePack to clone WebSocketRequest struct

* Use short form

* Fix test

model/websocket_request.go

@@ -4,31 +4,31 @@
 package model
 
 import (
-	"encoding/json"
-
 	"github.com/mattermost/mattermost-server/v6/shared/i18n"
+
+	"github.com/vmihailenco/msgpack/v5"
 )
 
 // WebSocketRequest represents a request made to the server through a websocket.
 type WebSocketRequest struct {
 	// Client-provided fields
-	Seq    int64                  `json:"seq"`    // A counter which is incremented for every request made.
-	Action string                 `json:"action"` // The action to perform for a request. For example: get_statuses, user_typing.
-	Data   map[string]interface{} `json:"data"`   // The metadata for an action.
+	Seq    int64                  `json:"seq" msgpack:"seq"`       // A counter which is incremented for every request made.
+	Action string                 `json:"action" msgpack:"action"` // The action to perform for a request. For example: get_statuses, user_typing.
+	Data   map[string]interface{} `json:"data" msgpack:"data"`     // The metadata for an action.
 
 	// Server-provided fields
-	Session Session            `json:"-"`
-	T       i18n.TranslateFunc `json:"-"`
-	Locale  string             `json:"-"`
+	Session Session            `json:"-" msgpack:"-"`
+	T       i18n.TranslateFunc `json:"-" msgpack:"-"`
+	Locale  string             `json:"-" msgpack:"-"`
 }
 
 func (o *WebSocketRequest) Clone() (*WebSocketRequest, error) {
-	buf, err := json.Marshal(o)
+	buf, err := msgpack.Marshal(o)
 	if err != nil {
 		return nil, err
 	}
 	var ret WebSocketRequest
-	err = json.Unmarshal(buf, &ret)
+	err = msgpack.Unmarshal(buf, &ret)
 	if err != nil {
 		return nil, err
 	}