Add GetFileLink method to plugin API (#9665)

* add GetFileLink method to plugin API

* Update plugin/api.go

* add translations for new plugin API errors

plugin/api.go

@@ -256,6 +256,11 @@ type API interface {
 	// Minimum server version: 5.3
 	GetFileInfo(fileId string) (*model.FileInfo, *model.AppError)
 
+	// GetFileLink gets the public link to a file by fileId.
+	//
+	// Minimum server version: 5.6
+	GetFileLink(fileId string) (string, *model.AppError)
+
 	// ReadFileAtPath reads the file from the backend for a specific path
 	//
 	// Minimum server version: 5.3