BUG/MEDIUM: h2/h3: reject some forbidden chars in :authority before reassembly

BUG/MEDIUM: h2/h3: reject some forbidden chars in :authority before reassembly

As discussed here:
   https://github.com/httpwg/http2-spec/pull/936
   https://github.com/haproxy/haproxy/issues/2941

It's important to take care of some special characters in the :authority
pseudo header before reassembling a complete URI, because after assembly
it's too late (e.g. the '/'). This patch does this, both for h2 and h3.

The impact on H2 was measured in the worst case at 0.3% of the request
rate, while the impact on H3 is around 1%, but H3 was about 1% faster
than H2 before and is now on par.

It may be backported after a period of observation, and in this case it
relies on this previous commit:

   MINOR: http: add a function to validate characters of :authority

Thanks to @DemiMarie for reviving this topic in issue #2941 and bringing
new potential interesting cases.