MINOR: h3: prepare support for response parsing

MINOR: h3: prepare support for response parsing

Refactor HTTP/3 request headers transcoding to HTX done in
h3_headers_to_htx(). Some operations are extracted into dedicated
functions, to check pseudo-headers and headers conformity, and also trim
the value of headers before encoding it in HTX.

The objective will be to simplify implementation of HTTP/3 response
transcoding by reusing these functions.

Also, h3_headers_to_htx() has been renamed to h3_req_headers_to_htx(),
to highlight that it is reserved to frontend usage.