FR: (messaging): add information about app instances & create registration tokens for APNs tokens

Please add ability to get information about app instances and create registration tokens for APNs tokens described in documentation of  "instance id server reference" there:
https://developers.google.com/instance-id/reference/server#get_information_about_app_instances
https://developers.google.com/instance-id/reference/server#create_registration_tokens_for_apns_tokens

