MINOR: http-htx: add a new generic trailers parser

MINOR: http-htx: add a new generic trailers parser

http_trailers_to_htx() merges h2_make_htx_trailers() and
h3_trailers_to_htx() into a unified function that takes a fields list,
performs controls and adds them to HTX. It returns http_parser_status
codes so that the caller knows what happens. For now the function is
not ready to handle HTTP/1 because it unconditionally rejects upper
case letters in field names.

The parser uses two modes to detect pseudo-headers:
  - syntax-based (it starts with a colon)
  - number-based (an index number replaces the string)

This is because the former is used by h3, the latter by h2.