Fix part of #5411: Introduce a simulated GAE proto endpoint [Blocked: #5415]

[BenHenning]

TODO

This 'simulates' the eventual endpoint that will be added to Oppia web by using internal compatibility checking, proto conversion, and the GAE/GCS JSON endpoints currently available via Oppia web.

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[oppiabot]

Hi @BenHenning, I'm going to mark this PR as stale because it hasn't had any updates for 7 days. If no further activity occurs within 7 days, it will be automatically closed so that others can take up the issue.
If you are still working on this PR, please make a follow-up commit within 3 days (and submit it for review, if applicable). Please also let us know if you are stuck so we can help you!