Add arbitrary sigs for lynx

[nsjames]

// Use await to return a promise
(async () => {

    let result;

    try {
        result = await window.lynxMobile.requestArbitrarySignature({
            data: "the string you wish to sign",
            whatFor: "The reason you want the signature. This will displayed to the user."
        });
    } catch (err) {
        console.log(err);
    }

    console.log(result); // "SIG_K1_JvbGYMSe7auMt8TvLUkoi1A1ywdqoNMmZ4vSeMfaFFtLL58eRGBtzk8HqbmyyrGWTUvhmFAQFHoMeF4icHrtwJQAqynqcD"

})();