Function save 

pub fn save(
    identity_id: &str,
    origin: &str,
    realm: &str,
    is_proxy: bool,
    username: &str,
    password: &str,
) -> Result<(), String>

Store (or overwrite) username/password for this identity + protection-space. Unbounded (no timeout) — see secret_store’s doc comment for why a write must not race a timed-out-but-later-completing keychain call.