Function build_linkrequest_data 

pub fn build_linkrequest_data(
    pub_bytes: &[u8; 32],
    sig_pub_bytes: &[u8; 32],
    mtu: Option<u32>,
    mode: LinkMode,
) -> Vec<u8> 

Build LINKREQUEST data: [x25519_pub:32][ed25519_pub:32][signalling:0-3].