Added the possibility of passing an external socket to HTTP20Connection

[ocampana]

ONVIF defines the Uplink service based on HTTP/2 and connection
reversal, in order to have cameras connect to cloud services while
having NAT between themselves and the remote service (For details,
https://www.onvif.org/specs/srv/uplink/ONVIF-Uplink-Spec.pdf)

With this patch, it is possible on the cloud side to accept an incoming
connection from a listining socket and to pass the new socket to
HTTP20Connection, so that the cloud software can use the reverted
connection and turn itself into a client.

Example code for implementing it:

import socket, ssl, time

from hyper import HTTP20Connection
from hyper.common.bufsocket import BufferedSocket

context = ssl.SSLContext (ssl.PROTOCOL_TLSv1_2)
context.load_cert_chain ("server.cert", "server.key")

bindsocket = socket.socket ()
bindsocket.bind(('', 8081))
bindsocket.listen(5)

with context.wrap_socket (bindsocket, server_side=True) as ssock:
    while True:
        newsocket, fromaddr = ssock.accept()

        req = newsocket.read ()

        if b'Connection: Upgrade' in req and b'Upgrade: h2c-reverse':
            newsocket.write (b'HTTP/1.1 101 Switching Protocols\r\nConnection: Upgrade\r\nUpgrade: h2c-reverse\r\n\r\n')
            newsocket.write (b'PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n')

            c = HTTP20Connection ('unused.org', external_socket = BufferedSocket (newsocket))

            c.request('POST', '/onvif/device_service', headers = { 'Content-Type': 'application/soap+xml; charset=utf-8'}, body=b'<?xml version="1.0" encoding="utf-8"?><soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:tds="http://www.onvif.org/ver10/device/wsdl" xmlns:tt="http://www.onvif.org/ver10/schema"><soap:Body><tds:GetDeviceInformation /></soap:Body></soap:Envelope>'))
            resp = c.get_response ()
            print (resp.read ())