Add a basic `step ssh verify` command

[hslatman]

This PR adds verify as an ssh subcommand. It verifies an SSH certificate using the ssh.CertChecker struct with basic options applied. Example output:

# failure with wrong CA key:
step ssh verify example-cert.pub wrong_ca_key.pub
ssh certificate signed by "SHA256:AXEctpST7/1MfakrLrE+xrtF8Eixh6YsmqNaxiN6AFI" does not equal ssh CA "SHA256:ezEEbt1V5MzJctHhrfk4ftfQMgOvPL51KaU/9MLouUo"
exit status 1

# success: 
step ssh verify example-cert.pub ca_key.pub

Some potential things to add/change:

• make CA key optional? verify would not check the signer key if not provided.
• allow multiple CA keys?
• add additional verification, such as revocation check?

[weaversam8]

One additional suggestion for this command: ability to add an optional --principal or --principals-file option that would check the principals in the certificate against an expected principal or a list of principals in a file (assuming the same format as the file that the AuthorizedPrincipalsFile directive supports within sshd)