http: opt-in insecure HTTP header parsing

Allow insecure HTTP header parsing. Make clear it is insecure.

See:
- https://github.com/nodejs/node/pull/30553
- https://github.com/nodejs/node/issues/27711#issuecomment-556265881
- https://github.com/nodejs/node/issues/30515

Backport-PR-URL: https://github.com/nodejs/node/pull/30471
PR-URL: https://github.com/nodejs/node/pull/30567


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>