Certificate chain in exchanges

[horo-t]

By using data-url in cert-url of Signature Header, the certificate chain can be contained in a Signed HTTP Exchange. It is useful for offline loading.

But having the same certificate chain in multiple Signed HTTP Exchanges in a Bundled HTTP Exchange is not efficient. We should have a way to contain one certificate chain per one host name for multiple Signed HTTP Exchanges in a Bundled HTTP Exchange.

I filed this issue from the discussion in https://crrev.com/c/1102281.