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

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.