MINOR: h3: support basic HTX start-line conversion into HTTP/3 request

MINOR: h3: support basic HTX start-line conversion into HTTP/3 request

This commit is the first one of a serie which aim is to implement
transcoding of a HTX request into HTTP/3, which is necessary for QUIC
backend support.

Transcoding is implementing via a new function h3_req_headers_send()
when a HTX start-line is parsed. For now, most of the request fields are
hardcoded, using a GET method. This will be adjusted in the next
following patches.