crypto: always accept private keys as public keys

Some APIs already accept private keys instead of public keys. This
changes all relevant crypto APIs to do so.

PR-URL: https://github.com/nodejs/node/pull/25217


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>