Notation key/cert list to support -o json output

[SteveLasker]

When looking to find the currently configured keys or certificates, there's no good way to get the current path as the config files may be in different locations on disk, or may not yet exist.

I may have a config with:

{
  "verificationCerts": {
    "certs": []
  },
  "signingKeys": {
    "default": "wabbit-networks.io",
    "keys": [
      {
        "name": "wabbit-networks.io",
        "path": "/home/stevelas/.config/notation/key/wabbit-networks.io.key"
      }
    ]
  },
  "insecureRegistries": [
    "registry.wabbit-networks.io"
  ]
}

I now want to add the wabbit-networks.io key to the verification certs collection. I don't know the path to the keys, or in this case, I actually need the certificate.

I was thinking of:

notation cert ls -o json

However, that would still give me the key, not the cert directory.

• Should keys and certs be in the same directory?
• Should notation cert directory and notation key location have an output?
• Should notation config location have an ouptput

[yizha1]

duplicated with #455, which was planned in v1.3.0 release.