http2: add altsvc support

Add support for sending and receiving ALTSVC frames.

PR-URL: https://github.com/nodejs/node/pull/17917


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>