GcmEx.Message
Message payload
Reference
https://developers.google.com/cloud-messaging/http-server-ref#downstream-http-messages-json
Summary
Types
t :: %GcmEx.Message{collapse_key: term, content_available: term, data: term, delay_while_idle: term, dry_run: term, notification: term, priority: term, registration_ids: term, restricted_package_name: term, time_to_live: term, to: term}