Default UserData impl can break if serde_json::to_string impl changes

[hu55a1n1]

Summary

The default UserData trait impl (i.e. with the derive macro), serializes to JSON and hashes that to get a commitment that is later included in remote attestations. The verifier uses the same code to check the user-data against the attestation. So a change in the serde_json::to_string() method could break RA for us.

Acceptance Criteria

• Use a more stable/canonical form of serialization for UserData.