MM-53924 - Implement NotificationWillBePushed plugin hook (#24263)

* add NotificationWillBePushed hook

* mocks

* use a struct for hook parameters; simplify number of parameters sent across RPC

* missing wg.Wait

* change to a bool return value

server/public/model/plugin_push_notification.go

@@ -0,0 +1,12 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+// PluginPushNotification is sent to the plugin when a push notification is going to be sent (via the
+// NotificationWillBePushed hook).
+type PluginPushNotification struct {
+	Post    *Post
+	Channel *Channel
+	UserID  string
+}