http2: add aligned padding strategy

Add a padding strategy option that makes a best attempt to ensure
that total frame length for DATA and HEADERS frames are aligned
on multiples of 8-bytes.

Backport-PR-URL: https://github.com/nodejs/node/pull/18050
Backport-PR-URL: https://github.com/nodejs/node/pull/20456
PR-URL: https://github.com/nodejs/node/pull/17938


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>