feat: add sponsored transaction support for gasless relay

[whoabuddy]

Summary

• Add sponsored?: boolean to PaymentDetails interface
• Thread sponsored flag through all transaction builders
• Add PaymentInterceptorConfig for axios interceptor
• When sponsored=true, transactions are built with fee: 0n for sponsor relay services

Usage

// Client-side: build sponsored transaction
const signedTx = await client.signSTXTransfer({
  ...details,
  sponsored: true
});

// Interceptor: configure for gasless
const api = withPaymentInterceptor(axios.create(), account, { sponsored: true });

Closes #7

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