Introducing x402-mcp: Open protocol payments for MCP tools

Introduces the x402 open protocol for programmatic payments in HTTP requests, enabling agents to interact with paid external services without prior account setup or key management. Implements `x402-mcp` to integrate x402 payments with Model Context Protocol (MCP) servers and the Vercel AI SDK. Provides `paymentMiddleware` for protecting API routes and `wrapFetchWithPayment` for client-side consumption. For MCP servers, `createPaidMcpHandler` allows defining `paidTool`s with prices. The protocol works by using the 402 status code for payment instructions, with clients submitting payment authorization in a header. Current implementations settle payments in USDC on the Base blockchain, but the standard is payment-network agnostic. A starter template demonstrates x402 with Next.js, AI SDK, AI Elements, and AI Gateway.