MINOR: h3: adjust auth request encoding or fallback to host

MINOR: h3: adjust auth request encoding or fallback to host

Implement proper encoding of HTTP/3 authority pseudo-header during
request transcoding on the backend side. A pseudo-header :authority is
encoded if a value can be extracted from HTX start-line. A special check
is also implemented to ensure that a host header is not encoded if
:authority already is.

A new function qpack_encode_auth() is defined to implement QPACK
encoding of :authority header using literal field line with name ref.