MINOR: h2: add h2_make_htx_trailers to turn H2 headers to HTX trailers

MINOR: h2: add h2_make_htx_trailers to turn H2 headers to HTX trailers

This function is usable to transform a list of H2 header fields to a
HTX trailers block. It takes care of rejecting forbidden headers and
pseudo-headers when performing the conversion. It also emits the
trailing CRLF that is currently needed in the HTX trailers block.