Add l402-go: Go client for L402 pay-per-request APIs

[joelklabo]

Summary

Adds l402-go - a Go client library for the L402 protocol (HTTP 402 + Lightning invoice + pay-and-retry).

What is L402?

L402 is a protocol for pay-per-request APIs using Lightning Network:

1. Client makes request
2. Server returns HTTP 402 with a Lightning invoice
3. Client pays invoice and retries with payment proof
4. Server fulfills request

The library provides a simple http.Client wrapper that handles the 402/retry flow automatically.

Example

client := l402.NewClient(wallet)
resp, err := client.Get("https://api.example.com/paid-endpoint")

This complements existing pay-per-call libraries like ln-paywall and paypercall by providing a standardized client-side implementation of the L402 protocol.