[native_rust_library] fix serialization of BackupKeysResult

Summary:
the validator in secondary-device-qr-code-scanner-react.js expects the field names to be camelCase

Depends on D12211

Test Plan:

const backupKeys = assertWithValidator<BackupKeys>(
  JSON.parse(backupKeysResponse),
  backupKeysValidator,
);

this assertion in secondary-device-qr-code-scanner-react.js was failing, but now succeeds

Reviewers: bartek, kamil

Reviewed By: bartek

Subscribers: marcin, ashoat, tomek

Differential Revision: https://phab.comm.dev/D12212