MINOR: h3: implement TRAILERS encoding

MINOR: h3: implement TRAILERS encoding

This patch implement the conversion of an HTX response containing
trailer into a H3 HEADERS frame. This is done through a new function
named h3_resp_trailers_send().

This was tested with a nginx configuration using <add_trailer>
statement.

It may be possible that HTX buffer only contains a EOT block without
preceeding trailer. In this case, the conversion will produce nothing
but fin will be reported. This causes QUIC mux to generate an empty
STREAM frame with FIN bit set.

This should be backported up to 2.7.