http2: add ping event

Add a Http2Session event whenever a non-ack PING is received.

Fixes: https://github.com/nodejs/node/issues/18514

/cc @murgatroid99 who requested the feature

/cc @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