feat(clients): implement the "clients" command

This command will return a list of clients registered with the server.
The command can be passed a --token option to skip fetching a new token.
Actually this is better as it's a valid `canGrant` clientId:

```
./bin/fxa-oauth.js --env=latest --user=rfkelly@mozilla.com token 5882386c6d801776 oauth
```

This works on master but fails out with "missing email" on this branch.