[native] rename fields in JSON serialized string to camelCase from snake_case

Summary: when we serialize objects in Rust we should use camelCase naming to match JS conventions

Test Plan: called registerUser from JS and the JSON string that I got back had the correct field names

Reviewers: bartek, kamil

Reviewed By: bartek, kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9127