MM-52804 - Implement SendPushNotification plugin api method (#24273)

* add SendPushNotification plugin api method

* lint; add testing.Short bc of the sleep

* add interface and generated layers

* add fields to PluginPushNotification; generate mocks

* SendPushNotification -> SendPluginPushNotification; improved comments

* more comments; fix test

* send api.ctx

server/public/plugin/hooks.go

@@ -291,6 +291,9 @@ type Hooks interface {
 	//
 	// To cancel a push notification, return true.
 	//
+	// Note that this method will be called for push notification sent by plugins, including
+	// the plugin that created the push notification.
+	//
 	// Minimum server version: 9.0
 	NotificationWillBePushed(pushNotification *model.PluginPushNotification) (cancel bool)
 }