Dry run

[kimbauters]

The FCM v1 API supports a validate_only flag: https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages/send . This is also known as a dry run in the Firebase Admin SDK. Messages that are sent with this flag enabled can effectively be used to test aspects such as network performance, while skipping the actual delivery of these messages to any devices.

This pull request adds the required field to the MessageWrapper, and introduces a new dry_run(...) function to the FcmClientBuilder to enable it. The default behaviour remains unchanged and will send messages through to devices. To use the dry run functionality you add dry_run(true):

let client = fcm::FcmClient::builder()
    ...
    .dry_run(true)
    .build()

The serialization is set up to skip the new field when it is set to false to ensure behaviour identical to the original implementation when dry_run(false) or when it is omitted.

[rj76]

Nice addition. Thanks! cargo fmt is failling though.

[kimbauters]

Indeed, there was a formatting issue. Should be fixed now.