HTTP Public Key Pinning (HPKP) is a security mechanism which allows HTTPS websites
to resist impersonation by attackers using mis-issued or otherwise fraudulent certificates.
(For example, sometimes attackers can compromise certificate authorities,
and then can mis-issue certificates for a web origin.)
The HTTPS web server serves a list of public key hashes, and on subsequent connections
clients expect that server to use 1 or more of those public keys in its certificate chain.
This commit will add this new functionality.
Fixes gh-3706
