MINOR: http/hardening: add "delete-all-trailer-fields"

MINOR: http/hardening: add "delete-all-trailer-fields"

Deletes from requests and responses any trailer field, regardless of
its content or syntax. This is done after syntactic checks though. The
vast majority of public facing applications do not use trailers because
they are not reliable (intermediaries are allowed to drop or fold them)
and their presence on public networks is generally associated with
attack attempts. It is generally safe to drop them by default on public
facing listeners, and even on servers meant to deliver public contents,
in order to avoid any information leakage. Note that gRPC services use
them.