feat(koa): introduce x402-koa payment middleware

[dtfiedler]

Implements Koa-compatible payment middleware that mirrors x402-express functionality while leveraging Koas native async/await capabilities for cleaner error handling and middleware flow.

Description

Tests

Checklist

• [x] I have formatted and linted my code
• [x] All new and existing tests pass
• [x] My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

[cb-heimdall]

🟡 Heimdall Review Status

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