MINOR: hq-interop: use zero-copy to transfer single HTX data block

MINOR: hq-interop: use zero-copy to transfer single HTX data block

Similarly to H3, hq-interop now uses zero-copy when dealing with a HTX
message with only a single data block. Exchange HTX and QCS buffer, and
use the HTX data block for HTTP payload. This is only possible if QCS
buffer is empty. Contrary to HTTP/3, no extra frame header is needed
before transferring HTTP payload.

hq-interop is only implemented for testing purpose so this change should
not be noticeable by users. However, it will be useful to be able to
test zero-copy transfer on QUIC interop testing.