Support HTTP/2 Cache-Digests and CASPer

[dunglas]

• HTTP/2 Cache-Digests: https://datatracker.ietf.org/doc/draft-ietf-httpbis-cache-digest/
• CASPer: https://h2o.examp1e.net/configure/http2_directives.html#http2-casper

Cache-Digests and Casper allows to cancels server-push if the client is known to be in possession of the content.

There is a Go library for CASPer (https://github.com/tcnksm/go-casper) but unfortunately it isn't compatible with how I implemented recursive pushes.