Support for importing public SSH keys from GitHub

Feature Request

I hope Ignition can support the import of public SSH keys from GitHub (or other trusted online identities).

To do that, we just need to call this opening API Endpoint:

$ curl -L https://api.github.com/users/mogeko/keys
[
  {
    "id": 64006488,
    "key": "ssh-rsa ..."
  }
]

Desired Feature

We can allow passwd.sshAuthorizedKeys to accept an identity Object:

{
  "passwd": {
    "users": {
      "name": "mogeko",
      "sshAuthorizedKeys": [
         { "server": "github", "user": "mogeko" },
         "other ssh keys..."
      ]
    }
  }
}

Or an identity String:

{
  "passwd": {
    "users": {
      "name": "mogeko",
      "sshAuthorizedKeys": [
        "gh:mogeko",
        "other ssh keys..."
      ]
    }
  }
}

Other Information

Here is a software with similar functions as a reference: ssh-import-id.

Start by locating the passwd.sshAuthorizedKeys handling and review the GitHub REST endpoint for listing a user's public keys. Use the object and string examples to define the accepted identity forms; the work is done when the configured identity resolves to imported public keys alongside existing SSH keys.