proposal: net/http: add a ServeConn method to http.Server to handle net.Conn


```
// x/net/http
func (s *Server) ServeConn(c net.Conn, opts *ServeConnOpts) 
```

or

```
// github.com/valyala/fasthttp
func (s *Server) ServeConn(c net.Conn) error 
```