Summary: This makes an equivalent of the /records REST endpoint available within custom widgets. For simple operations, it is compatible with https://github.com/airtable/airtable.js/. About half of the diff is refactoring code from DocApi that implements /records using applyUserActions, to make that code available in the plugin api.
Test Plan: added tests
Reviewers: alexmojaki
Reviewed By: alexmojaki
Differential Revision: https://phab.getgrist.com/D3320
