BUG/MINOR: h3: Set HTX flags corresponding to the scheme found in the request

BUG/MINOR: h3: Set HTX flags corresponding to the scheme found in the request

When a ":scheme" pseudo-header is found in a h3 request, the
HTX_SL_F_HAS_SCHM flag must be set on the HTX message. And if the scheme is
'http' or 'https', the corresponding HTX flag must also be set. So,
respectively, HTX_SL_F_SCHM_HTTP or HTX_SL_F_SCHM_HTTPS.

It is mainly used to send the right ":scheme" pseudo-header value to H2
server on backend side.

This patch could be backported as far as 2.6.