doctl auth list: respect json output flag

[loosla]

addresses #1543

Updates the doctl auth list command to respect the -o json flag.

Example output:

$ go run cmd/doctl/main.go auth list -o json

[
  {
    "name": "default",
    "current": true
  },
  {
    "name": "personal",
    "current": false
  },
  {
    "name": "work",
    "current": false
  }
]

[bentranter]

Looks great! Thanks so much for the contribution Anna!