Rework public key

[stefanberger]

This PR reworks the usage of the public keys.

• We extend the YAML file with support for pkcs11 module specific environment variables that we already set before the usage of the pkcs11 module. We now require these types of YAML files also to be used for public keys since the YAML is more expressive than the plain URI.
• Relax the requirements for a PIN. At least for softhsm2 public key access only a session is needed but no login is required.

[lumjjb]

Linking issue: https://github.com/containers/ocicrypt/issues/20