User guptas custom json marshaller

[ns-guptas]

I have:

• [ ] Written a clear PR title and description (above)
• [ ] Signed the Khan Academy CLA
• [ ] Added tests covering my changes, if applicable
• [ ] Included a link to the issue fixed, if applicable
• [ ] Included documentation, for new features
• [ ] Added an entry to the changelog

[benjaminjkraft]

Thanks for your interest in genqlient! This doesn't look like something we can merge but feel free to open an issue if you have any questions.