This patch adds a "early_hint" struct to "arg" union of "act_rule" struct
and parse "early-hint" http-request keyword with it using the same
code as for "(add|set)-header" parser.
These ones are mostly called from cfgparse.c for the parsing and do
not depend on the HTTP representation. The functions's prototypes
were moved to proto/http_rules.h, making this file work exactly like
tcp_rules. Ideally we should stop calling these functions directly
from cfgparse and register keywords, but there are a few cases where
that wouldn't work (stats http-request) so it's probably not worth
trying to go this far.