Add x402 protocol and xPay to Infrastructure

[sriakula1]

Hi! I'd like to add two entries to the Infrastructure section:

• x402 — an open protocol implementing HTTP 402 Payment Required for machine-to-machine payments using USDC on Base. The awesome-x402 repo catalogs MCP servers, middleware, and developer tools built on it.
• xPay — AI agent payment infrastructure built on Base, enabling autonomous agents to transact via the x402 protocol.

Both are Base-native projects — x402 uses USDC on Base as its primary payment rail. Entries are in alphanumeric order matching the existing format.

Thank you for maintaining this list!