Add support for ed25519 key in S/MIME certificate

[rrrooommmaaa]

Make sure ed25519 public and private keys are correctly loaded for usage with S/MIME certificates, as well as encrypted for secure storing.

Originally posted by @tomholub in #3860 (comment)

[rrrooommmaaa]

Also test matching the private key with the certificate, during parsing and/or decryption, where applicable.