MEDIUM: mux-h2: add basic H2->HTX transcoding support for headers

MEDIUM: mux-h2: add basic H2->HTX transcoding support for headers

When the proxy is configured to use HTX mode, the headers frames
will be converted to HTX header blocks instead of HTTP/1 messages.
This requires very little modifications to the existing function
so it appeared better to do it this way than to duplicate it.

Only the request headers are handled, responses are not processed
yet and data frames are not processed yet either. The return value
is inaccurate but this is not an issue since we're using it as a
boolean : data received or not.