Proxy client serializes requests on a single connection (no concurrency)

[ameba23]

This is a performance issue, and a proposed enhancement to increase throughput.

Problem

ProxyClient::new_with_inner forwards all incoming requests through a single sender in a single async loop. Each request awaits sender.send_request(req) before the next is handled, which serializes all proxy traffic.

Impact

• Throughput is capped by per‑request round‑trip time.
• Head‑of‑line blocking under concurrency.
• Even with HTTP/2 negotiated, the implementation does not drive multiplexed concurrency.

Suggested fix

Enable concurrent sending:

• For HTTP/2, clone the sender and dispatch requests concurrently (bounded).
• For HTTP/1, consider multiple outbound connections or a small pool to parallelize.
• Keep a per‑connection concurrency limit to avoid overload.

Acceptance criteria

• Multiple concurrent inbound requests do not block each other.
• Throughput increases in a concurrent load test (e.g., 50–200 in‑flight requests).
• No regression in response integrity or attestation metadata injection.