Function decode_phc 

pub fn decode_phc(
    encoded: &str,
) -> Result<(Params, Vec<u8>, Vec<u8>), Argon2Error>

Decode an Argon2id PHC string format into (params, salt, tag).

Expected format: $argon2id$v=19$m=<m>,t=<t>,p=<p>$<salt_b64>$<hash_b64>