MINOR: h3: transcode H3 response headers into HTX blocks

MINOR: h3: transcode H3 response headers into HTX blocks

Finalize HTTP/3 response transcoding into HTX message. This patch
implements conversion of HTTP/3 headers provided by the server into HTX
blocks.

Special checks have been implemented to reject connection-specific
headers, causing the stream to be shut in error. Also, handling of
content-length requires that the body size is equal to the value
advertized in the header to prevent HTTP desync.