x402: add payWithX402 method to SharpyClient for agentic invoice payments

[MarcusDavidG]

x402 enables AI agents and HTTP clients to pay Sharpy invoices without a wallet UI by using signed Soroban auth entries.

Implementation:

• Install x402-stellar npm package
• Add payWithX402(invoiceId: number, payerAddress: string, facilitatorUrl: string) to SharpyClient
• Add x402 facilitator URLs to NETWORKS constant (Coinbase testnet, OpenZeppelin mainnet)
• Export x402 helpers from SDK index

Reference: https://developers.stellar.org/docs/build/agentic-payments/x402
npm: https://www.npmjs.com/package/x402-stellar