crypto: add docs & tests for cert.pubkey & cert.fingerprint256

Include example on how to pin certificate and/or public key

PR-URL: https://github.com/nodejs/node/pull/17690


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>