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/api.go

@@ -1180,6 +1180,15 @@ type API interface {
 	// @tag Upload
 	// Minimum server version: 7.6
 	GetUploadSession(uploadID string) (*model.UploadSession, error)
+
+	// SendPluginPushNotification will attempt to send a push notification to `notification.User`, using
+	// `notification.Post` as the source of the notification. The server will use the PluginPushNotification
+	// data to construct the final push notification according to the server's configuration and license. Refer
+	// to `App.BuildPushNotificationMessage` for the logic used to construct the push notification.
+	// Note: the NotificationWillBePushed hook will be run after SendPluginPushNotification is called.
+	//
+	// Minimum server version: 9.0
+	SendPluginPushNotification(notification *model.PluginPushNotification) error
 }
 
 var handshake = plugin.HandshakeConfig{