MEDIUM: mux-h2: implement encoding of H2 request on the backend side

MEDIUM: mux-h2: implement encoding of H2 request on the backend side

This creates an H2 HEADERS frame from an HTX request. The code is
very similar to the response encoding, so probably that in the future
we'll have to factor these functions differently. The HTX's start line
type is used to decide on the direction. We also purposely error out
when trying to encode an H2 request from an H1 message since it's not
implemented.