MEDIUM: httpclient: enable ALPN support on outgoing https connections

MEDIUM: httpclient: enable ALPN support on outgoing https connections

Since everything is available for this, let's enable ALPN with the
usual "h2,http/1.1" on the https server. This will allow HTTPS requests
to use HTTP/2 when available.

It may be needed to permit to disable this (or to set the string) in
case some client code explicitly checks for the "HTTP/1.1" string, but
since httpclient is quite young it's unlikely that such code already
exists.