feat: make HttpProxy public

[ma-ts]

I am currently working on a use-case to use Pingora to provide both TLS passthrough and as a HTTP proxy / load balancer. For this, what I need to do is accept a TLS session, do an SNI peek, and then make the decision of whether or not I want to provide it to the HTTP proxy, or just do a direct passthrough, something like this:

use pingora_proxy::http_proxy;
use pingora_core::apps::HttpServerApp;
use std::sync::Arc;

let proxy = Arc::new(http_proxy(&server_conf, my_proxy_app));
loop {
    let (stream, addr) = listener.accept().await?;
    
    // check SNI host, make decision
    
    if should_terminate_tls {
        let tls_stream = my_acceptor.accept(stream).await?;
        let session = HttpSession::new_http1(Box::new(tls_stream));
        proxy.process_new_http(session, &shutdown).await;
    }
}

However, currently HttpSession is not public, which makes this very difficult for this use-case. Would it be possible to open this up?