Fix part of #5411: Add support for JSON->proto conversion [Blocked: #5413]

[BenHenning]

TODO: Finish.

This is the basis for converting from GAE JSON structures to proto "v2" (that is, the proto structures defined as part of oppia-proto-api). "v1" conversion (that is, the proto structures used in Oppia Android's data layer) will happen at a higher level in a downstream PR.

Explanation

Essential Checklist

• [ ] The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• [ ] Any changes to scripts/assets files have their rationale included in the PR explanation.
• [ ] The PR follows the style guide.
• [ ] The PR does not contain any unnecessary code changes from Android Studio (reference).
• [ ] The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• [ ] The PR is assigned to the appropriate reviewers (reference).

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[oppiabot[bot]]

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!