http connect tunnel support

[chlxt]

Currently tiny-http may not support CONNECT method well. After the first CONNECT event occurs, all subsequent DATA from peer host should not be treated as Request anymore, so tiny-http should expose corresponding socket(or reader/writer) to the user and remove this connection from contentiously receiving loop thread.

two options I've thought:

1. changing the interface: incoming_requests() returns new Message type:

   enum Message {
       Error(IoError),
       NewRequest(Request),
   }

   to

   enum Message {
       Error(IoError),
       NewRequest(Request),
       NewData(Vec<u8>),
   }

   and in receiving loop check the message type first, if is CONNECT then keeps track of this connection and doesn't do anymore Reqeust parsing on any subsequent packets.

   let mut is_normal_req = false;
   for rq in client {
       if is_normal_req {
           if(rq.method() == Method::CONNECT) {
               is_normal_req = false;
           }
           todo!(); // push `rq` to queue `messages`
       } else {
           todo!(); // directly return DATA(Vec<u8>)
       }
   }
   

2. changing the CONNECT related data, when CONNECT arrives, returns CONNECT Request with the underlying sock(or reader/writer) to users, and break the receiving loop thread.

   for rq in client {
       let is_normal_req = rq.method == Method::CONNECT;
       todo!(); // push `rq` to queue `messages`, and attach sock(or reader/writer) to this Reqeust Message if is `CONNECT`
       if is_normal_req {
           break;
       }
   }
   

   now users can reuse returned sock(or reader/writer) to construct new TCP tunnel.

any suggestions?