Function start_login 

pub async fn start_login<S: PasskeyStore + ?Sized>(
    store: &S,
    config: &PasskeyConfig,
    now_ms: i64,
) -> Result<PublicKeyCredentialRequestOptions>

Initiates a passkey login flow.

Returns the options that must be sent to the WebAuthn client (navigator.credentials.get). It saves a login session state keyed by the challenge.