Extract SHA256 Hashes of Public Keys for the User

[paragonie-scott]

While it's possible for users to extract public keys and calculate a SHA256 hash and then invoke addHash() on an opaque string constant, we should consider adding support for extracting public keys from:

• Local TLS certificates signed by a CA
• Certificate Signing Requests
• Remote TLS certificates on a web URL

This gives us a few added benefits:

• With Caddy, you can refresh the "active" certificate which only lives for 90 days (LetsEncrypt)
• CloudFlare users can send an HTTP request and get the certificate in use by the edge server

This is, in my opinion, a usability win.