Add sponsored transaction support

[whoabuddy]

Summary

Add support for sponsored (gasless) transactions to enable relay services that cover gas fees for agents.

Changes Needed

1. Add sponsored?: boolean to PaymentDetails interface in types.ts
2. Pass sponsored: true flag to makeSTXTokenTransfer() and makeContractCall() in:
   • client.ts: signSTXTransfer, sendSTXTransfer, signSBTCTransfer, sendSBTCTransfer, signUSDCxTransfer, sendUSDCxTransfer
   • interceptor.ts: signPayment

Usage

// Agent builds sponsored tx (no fee)
const signedTx = await client.signPayment(paymentDetails, { sponsored: true });

// Sponsor relay adds fee and broadcasts
const sponsoredTx = await sponsorTransaction({ transaction, sponsorPrivateKey, fee });

Context

Enables x402 sponsor relay pattern where agents submit pre-signed transactions and a relay service covers the gas fees.