Allow to use certificates as key parameters

[maraino]

Description

Some commands like step crypto jwt verify should accept certificates as --key parameters, we can easily extract a public key from it and verify the JWT.

Related to #67

[dopey]

Added the ability to use X509 Certificates in place of public keys on the command line (where public keys are required). A common use case for this flexibility can be found in the step crypto subcommands where you may need to provide a public key to verify a signature but the public key is stored as an X509 Certificate.