MEDIUM: h2: parse Extended CONNECT request to htx

MEDIUM: h2: parse Extended CONNECT request to htx

Support for the rfc 8441 Bootstraping WebSockets with HTTP/2

Convert an Extended CONNECT HTTP/2 request into a htx representation.
The htx message uses the GET method with an Upgrade header field to be
fully compatible with the equivalent HTTP/1.1 Upgrade mechanism.

The Extended CONNECT is of the following form :

:method = CONNECT
:protocol = websocket
:scheme = https
:path = /chat
:authority = server.example.com

The new pseudo-header :protocol has been defined and is used to identify
an Extended CONNECT method. Contrary to standard CONNECT, Extended
CONNECT must have :scheme, :path and :authority defined.