Get a single Device Registration
This operation fetches a single Device Registration.
* A Device Registration contains a list of devices registered for a particular `User` under an `App`
* The `Device` tokens in the Device Registration are used for delivering Push Notifications to the given `App` on
devices or browsers belonging to a single User.
* Each User will have one Device Registration for each `App`.
* When sending Push Notifications to a User, all of the
devices registered under the Device Registration with the corresponding `appName`
are targets for the Push Notification.