Abstraction on top of Oppia GAE

[BenHenning]

The Oppia GAE backend needs to have an abstraction from which the rest of the app can interact. Since the prototype and MVPs are not expected to support push notifications, this interface will be entirely RESTful with async/future responses with error handling support. This implementation should support RPC retry for appropriate 5xx errors.

Work items:

• Identify among all existing GAE endpoints (get/post), which we can use during the prototype
• Loosely identify data missing from the above endpoints (this bucket is expected to be small/non-existent), and identify which endpoints have data which needs to be changed for the Android app
• Specify the Retrofit interfaces to represent all of the identified endpoints (simple interfaces/no annotations or return types needed beyond pseudocode)
• Identify what data (via pseudocode data structures/YAML/JSON/etc) we will be passing along from the backend through these endpoints
• Identify how error handling will work (e.g. what Retrofit does), whether RPC retry is supported & how it is/can be configured, etc.
• Determine the testing strategy for the GAE endpoints for downstream app components

[BenHenning]

Design document: https://docs.google.com/document/d/1Ppnr-cph-Y0E2kGlHmYlCM6VWsPn2lGDlLGhcYeflgU/edit?usp=sharing

[BenHenning]

This is blocked on #4 for implementation.

[BenHenning]

I split out the Retrofit bits to #88, so this is now blocked on that. #88 is targeting proof-of-concept and the rest of this really should be part of prototype.