http2: add altsvc support

Add support for sending and receiving ALTSVC frames.

As defined by https://tools.ietf.org/html/rfc7838

ping @nodejs/http2

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

http2