MEDIUM: mux_h2: generate Extended CONNECT from htx upgrade

MEDIUM: mux_h2: generate Extended CONNECT from htx upgrade

Support for the rfc 8441 Bootstraping WebSockets with HTTP/2

Generate an HTTP/2 Extended CONNECT request from a htx Upgrade message.
This conversion is done when seeing the header Connection: Upgrade. A
CONNECT request is written with the :protocol pseudo-header set from the
Upgrade htx header value.

The protocol is saved in the h2s structure. This is needed on the
response side because the protocol is not present on HTTP/2 response but
is needed if the client side is using HTTP/1.1 with 101 status code.