http: make parser choice a runtime flag

Add a `--http-parser=llhttp` vs `--http-parser=traditional`
command line switch, to make testing and comparing the new
llhttp-based implementation easier.

PR-URL: https://github.com/nodejs/node/pull/24739
Refs: https://github.com/nodejs/node/issues/24730


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>