MM-42810: Introduce a channel hook for a websocket event (#23812)

Sometimes a broad distinction of just a channelID or a userID
is not enough to efficiently send a websocket event to users.

In several cases, depending on the user and channel, we might
need to modify the message. Therefore, we introduce the
concept of a channel hook that will get executed if the scope
is set to a channel. This hook can be populated at the app layer
to perform any application specific logic to the event.

Care must be taken to avoid race conditions as the passed event
is not deep copied. It is left to the user to treat it carefully.

For this issue, the main problem was that since we don't know
which users have permissions to which channels, we had to go through
_all_ members of a channel to figure that out. This was redundant
since a large portion of those users might not even be connected
at that time.

We solve this with the channel hook where we push this check
to be performed later while actually sending the event. This
reduces the computation to be done only for _connected_ users
rather than _all_ users of a channel.

The next iteration of this should be to use websocket subscriptions
to monitor exactly which users are on that channel to even
trim down that list. That is a larger initiative to be taken later.

Tested locally with a channel of 50 users. Here are rough results:
```
With PR:
patchPost 97ms
createPost 90ms

Master:
patchPost 306ms
createPost - 298ms
```

https://mattermost.atlassian.net/browse/MM-42810

```release-note
Improve  performance while sending messages
with permalinks to channels with large number of users.
```

server/public/model/websocket_message.go

@@ -92,14 +92,15 @@ type WebSocketMessage interface {
 }
 
 type WebsocketBroadcast struct {
-	OmitUsers             map[string]bool `json:"omit_users"`                        // broadcast is omitted for users listed here
-	UserId                string          `json:"user_id"`                           // broadcast only occurs for this user
-	ChannelId             string          `json:"channel_id"`                        // broadcast only occurs for users in this channel
-	TeamId                string          `json:"team_id"`                           // broadcast only occurs for users in this team
-	ConnectionId          string          `json:"connection_id"`                     // broadcast only occurs for this connection
-	OmitConnectionId      string          `json:"omit_connection_id"`                // broadcast is omitted for this connection
-	ContainsSanitizedData bool            `json:"contains_sanitized_data,omitempty"` // broadcast only occurs for non-sysadmins
-	ContainsSensitiveData bool            `json:"contains_sensitive_data,omitempty"` // broadcast only occurs for sysadmins
+	ConnectionId          string                                       `json:"connection_id"`                     // broadcast only occurs for this connection
+	OmitConnectionId      string                                       `json:"omit_connection_id"`                // broadcast is omitted for this connection
+	UserId                string                                       `json:"user_id"`                           // broadcast only occurs for this user
+	OmitUsers             map[string]bool                              `json:"omit_users"`                        // broadcast is omitted for users listed here
+	ChannelId             string                                       `json:"channel_id"`                        // broadcast only occurs for users in this channel
+	ChannelHook           func(userID string, ev *WebSocketEvent) bool `json:"-"`                                 // ChannelHook is a function that runs for a channel scoped event. It can be used to modify the event payload based on some custom logic that runs only for connected users. The return value indicates whether the websocket event was modified or not.
+	TeamId                string                                       `json:"team_id"`                           // broadcast only occurs for users in this team
+	ContainsSanitizedData bool                                         `json:"contains_sanitized_data,omitempty"` // broadcast only occurs for non-sysadmins
+	ContainsSensitiveData bool                                         `json:"contains_sensitive_data,omitempty"` // broadcast only occurs for sysadmins
 	// ReliableClusterSend indicates whether or not the message should
 	// be sent through the cluster using the reliable, TCP backed channel.
 	ReliableClusterSend bool `json:"-"`
@@ -189,10 +190,21 @@ func (ev *WebSocketEvent) PrecomputeJSON() *WebSocketEvent {
 	return evCopy
 }
 
+func (ev *WebSocketEvent) RemovePrecomputedJSON() {
+	ev.precomputedJSON = nil
+}
+
 func (ev *WebSocketEvent) Add(key string, value any) {
 	ev.data[key] = value
 }
 
+// AddWithCopy copies the map and writes to a copy of that,
+// and sets the map to the new event.
+func (ev *WebSocketEvent) AddWithCopy(key string, value any) {
+	ev.data = copyMap(ev.data)
+	ev.data[key] = value
+}
+
 func NewWebSocketEvent(event, teamId, channelId, userId string, omitUsers map[string]bool, omitConnectionId string) *WebSocketEvent {
 	return &WebSocketEvent{
 		event: event,
@@ -218,17 +230,9 @@ func (ev *WebSocketEvent) Copy() *WebSocketEvent {
 }
 
 func (ev *WebSocketEvent) DeepCopy() *WebSocketEvent {
-	var dataCopy map[string]any
-	if ev.data != nil {
-		dataCopy = make(map[string]any, len(ev.data))
-		for k, v := range ev.data {
-			dataCopy[k] = v
-		}
-	}
-
 	evCopy := &WebSocketEvent{
 		event:           ev.event,
-		data:            dataCopy,
+		data:            copyMap(ev.data),
 		broadcast:       ev.broadcast.copy(),
 		sequence:        ev.sequence,
 		precomputedJSON: ev.precomputedJSON.copy(),
@@ -236,6 +240,14 @@ func (ev *WebSocketEvent) DeepCopy() *WebSocketEvent {
 	return evCopy
 }
 
+func copyMap[K comparable, V any](m map[K]V) map[K]V {
+	dataCopy := make(map[K]V, len(m))
+	for k, v := range m {
+		dataCopy[k] = v
+	}
+	return dataCopy
+}
+
 func (ev *WebSocketEvent) GetData() map[string]any {
 	return ev.data
 }