equip etrecord class with save method

Pull Request resolved: #12978

design doc: #12925
This diff creates a save function for etrecord class for export flow usage. Also make the codebase more structural to make lintrunner happy.
ghstack-source-id: 299603283

Differential Revision: [D79205242](https://our.internmc.facebook.com/intern/diff/D79205242/)

Author: Gasoonjia