Add support for keys retrieval with keys_jwk

[Rémy HUBSCHER]

const fxaKeyUtils = new FxaCryptoRelier.KeyUtils();

function handleAuthentication() {
  return fxaKeyUtils.createApplicationKeyPair()
    .then((keyTypes) => {
        return {url: `${AUTH_URL}&keys_jwk=${keyTypes.base64JwkPublicKey}`};
      });
    })