function encodePayment(payment: {
  network: string;
  payload:
    | {
        authorization: {
          from: string;
          nonce: string;
          to: string;
          validAfter: string;
          validBefore: string;
          value: string;
        };
        signature: string;
      }
    | { transaction: string };
  scheme: "exact";
  x402Version: number;
}): string;