Support conversion of Key object to OpenSSL-ish object

[grzuy]

I assume most of users deserializing COSE Key objects, would need as a follow up step to operate with that key, meaning, signing, verifying a signature, encrypting or decrypting a message.

Given that, it would be very useful for the Key object to return an instance of a ready-to-operate ruby-openssl OpenSSL::PKey::PKey object.

[grzuy]

If we support this here, then e.g. this would not be needed in webauthn-ruby.

[grzuy]

By OpenSSL-ish object I mean an instance of OpenSSL::PKey::PKey