Proxy server opens a new outbound TCP/HTTP connection per request (no reuse)

This is a performance issue and a proposal to enhance performance / throughput.

## Problem
`ProxyServer::handle_http_request` opens a fresh `TcpStream::connect` and performs a new Hyper HTTP/1.1 client handshake for **every** incoming request. This disables keep‑alive reuse and adds per‑request connection overhead (DNS resolution, TCP handshake, HTTP handshake).

- File: `src/lib.rs`
- Function: `ProxyServer::handle_http_request`

## Impact
- Throughput is limited by connection setup rather than request handling.
- Higher latency under load.
- Increased CPU usage and ephemeral port churn.

## Proposed fix
Introduce outbound connection reuse:
- Maintain a persistent HTTP client connection to the target service.
- Prefer HTTP/2 upstream when available to multiplex requests.
- Avoid `TcpStream::connect` per request.

## Acceptance criteria
- Outbound connections are reused across multiple requests (observable via logs).
- Nice to have: throughput improves in a load test vs. current baseline.
- No regression in proxy correctness (headers, attestation metadata, error handling).

## Notes
The current code creates the outbound connection here:
- `src/lib.rs` → `ProxyServer::handle_http_request` (`TcpStream::connect`, `http1::Builder::handshake`).