Add ssh-key add command and publish ssh-key

[mislav]

$ gh ssh-key add ~/.ssh/id_ed25519.pub
✓ Public key added to your account

$ gh ssh-key
Manage SSH keys registered with your GitHub account

USAGE
  gh ssh-key <command> [flags]

CORE COMMANDS
  add:        Add an SSH key to your GitHub account
  list:       Lists SSH keys in your GitHub account

Followup to #2892, #1862
Ref. #1755

[vilmibm]

tested locally included auth refreshes, looks great!