feat: use nym hash as probe_id for use on clients

[DecFox]

This diff allows us to expose the nym hash as the probe_id to be used for stamping ooni measurements. We also cleanup the redundant code we had for exposing stateless functions for client integration.

[mmaker]

can we rely on the object-oriented function approach?

[DecFox]

can we rely on the object-oriented function approach?

Thanks! I reverted the changes to using object-oriented approach for the UserState as well.

I also added this setter function: https://github.com/ooni/userauth/pull/22/changes#diff-c0b83d506440f65511f38f3ff8e8aeb659d2c7b04e400762950d2eac92f1146aR84, to set the stored credential from the clients before we make the rust function calls. Do you think this is good or a better way is to store the entire UserState as is, with the public params in the clients (for now, we only store the base64-encoded credential)?

[DecFox]

Another question would be, with the digest_point function now here, can we also make it public? This should allow us to extract the probe_id on the clients through the saved credential on the device given a probe_asn and a probe_cc. The probe can then decide to attach it inside the measurement (example implementation in the client attached below):

pub fn get_probe_id(
    credential_b64: String,
    probe_asn: String,
    probe_cc: String
) -> Result<ProbeIDResult, OoniError> {
    let credential = decode_credential(&credential_b64)?;
    let domain_str = format!("ooni.org/{}/{}", probe_cc, probe_asn);
    let DOMAIN = G::hash_from_bytes::<Sha512>(domain_str.as_bytes());
    let NYM = credential.nym_id.unwrap() * DOMAIN;

    let probe_id = ooniauth_core::digest_point(NYM)

    OK(ProbeIDResult {
        probe_id: probe_id
    })
}

Or maybe extract away the entire nym processing and hashing to a dedicated function here and use it off the plate in the client?