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/plugin/hooks.go

@@ -50,6 +50,7 @@ const (
 	deprecatedGetCollectionMetadataByIdsID    = 32
 	deprecatedGetTopicMetadataByIdsID         = 33
 	ConfigurationWillBeSavedID                = 34
+	NotificationWillBePushedID                = 35
 	TotalHooksID                              = iota
 )
 
@@ -284,4 +285,12 @@ type Hooks interface {
 	// config object can be returned to be stored in place of the provided one.
 	// Minimum server version: 8.0
 	ConfigurationWillBeSaved(newCfg *model.Config) (*model.Config, error)
+
+	// NotificationWillBePushed is invoked before a push notification is sent to the push
+	// notification server. The intention is to allow plugins to cancel a push notification.
+	//
+	// To cancel a push notification, return true.
+	//
+	// Minimum server version: 9.0
+	NotificationWillBePushed(pushNotification *model.PluginPushNotification) (cancel bool)
 }