MEDIUM: mux-h2: emit HEADERS frames when facing HTX trailers blocks

MEDIUM: mux-h2: emit HEADERS frames when facing HTX trailers blocks

Now the H2 mux will parse and encode the HTX trailers blocks and send
the corresponding HEADERS frame. Since these blocks contain pure H1
trailers which may be fragmented on line boundaries, if first needs
to collect all of them, parse them using the H1 parser, build a list
and finally encode all of them at once once the EOM is met. Note that
this HEADERS frame always carries the end-of-headers and end-of-stream
flags.

This was tested using the helloworld examples from the grpc project,
as well as with the h2c tools. It doesn't seem possible at the moment
to test tailers using varnishtest though.