feat(wangapayfast-rs): add axum middleware for verified ITN handling

[mitch1009]

Summary

Provide a reusable middleware/extractor layer for framework integration with verified ITN payloads.

Scope

• Add axum middleware or extractor for parse+verify flow
• Return normalized error responses on invalid payload/signature
• Include sample route integration

Acceptance Criteria

• Users can wire verified ITN handling with minimal boilerplate
• Invalid ITN requests are rejected consistently
• Integration tests cover middleware behavior