turnloop-http: support https:// proxies (TLS to the proxy, then CONNECT) #111

turnloop_http::client::ProxyEnvironment::proxy_for refuses any proxy whose scheme is not http (turnloop-http 0.1.0-alpha.6, src/client.rs:215-220), so an https:// proxy (TLS to the proxy itself, then CONNECT, then TLS to the origin inside it) cannot be expressed.

Perry's fetch previously fell back to reqwest for this case, which supports HTTPS proxies. PerryTS/perry#11101 deletes that fallback, so such a fetch now rejects with only HTTP proxies are supported.

Needed:

1. proxy_for accepts https proxy URIs and reports the scheme to the caller.
2. A way to layer a turnloop-tls client session over an already-TLS stream (TLS-in-TLS) for the CONNECT tunnel, or guidance on how the embedder should compose it.

No test in Perry covers this today; it is a regression relative to the reqwest path, not a Node-parity test failure.