Function client_first_message 

pub fn client_first_message(username: &str) -> (String, String)

Client-side: Create the client-first-message data (base64-encoded).

Returns (client_nonce, client_first_data_base64).

The client-first-message-bare is n=<username>,r=<client_nonce>. The full message prepends the GS2 header n,, (no channel binding).