Derive Debug for ProxyStream

[ptwales]

Hyper-boring's HttpsConnector requires the Response type to implement Debug. Since ProxyStream doesn't implement Debug one cannot create a HttpsConnector<ProxyStream<HttpConnector>> with Boring SSL. Compilation will fail with this error

error[E0277]: `ProxyStream<tokio::net::tcp::stream::TcpStream>` doesn't implement `std::fmt::Debug`
   --> src/lib.rs:75:17
    |
75  |     let https = HttpsConnector::with_connector(proxied, ssl)?;
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ProxyStream<tokio::net::tcp::stream::TcpStream>` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
    |
    = help: the trait `std::fmt::Debug` is not implemented for `ProxyStream<tokio::net::tcp::stream::TcpStream>`
note: required by `HttpsConnector::<S>::with_connector`
   --> /home/ptwales/.cargo/registry/src/github.com-1ecc6299db9ec823/hyper-boring-2.1.1/src/lib.rs:176:5
    |
176 | /     pub fn with_connector(
177 | |         http: S,
178 | |         ssl: SslConnectorBuilder,
179 | |     ) -> Result<HttpsConnector<S>, ErrorStack> {
    | |______________________________________________^

It's debatable whether the change should go here or in cloudflare/boring/hyper-boring. Hyper-boring is forked from hyper-openssl and while hyper-openssl did originally have that Debug requirement, they dropped it when they upgraded to hyper v0.14. Hyper-boring is also on v0.14 but didn't remove that requirement.

I'm requesting the change be implemented here in hyper-proxy because it's a one line change that won't break anything, whereas it could be many lines and many possible breakages downstream if it was in hyper-boring.