Convert APNs tokens to Firebase registration tokens

[halaei]

Describe the feature you would like to see

I just wondering if you find it fit to add batchImport method to this library. The API converts APNs tokens to Firebase tokens. I hope it will be a handy feature for iOS app developers, since the iOS devices give us APNs tokens instead of Firebase messaging tokens, which need to be converted in server side.

The API reference: https://developers.google.com/instance-id/reference/server#create_registration_tokens_for_apns_tokens

Using the Instance ID service's batchImport method, you can bulk import existing iOS APNs tokens to Google Cloud Messaging or Firebase Cloud Messaging, mapping them to valid registration tokens. Call the Instance ID service at this endpoint, providing a list of APNs tokens in the JSON body:
https://iid.googleapis.com/iid/v1:batchImport
The response body contains an array of Instance ID registration tokens ready to be used for sending FCM or GCM messages to the corresponding APNs device token.

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[ni3k]

I already have an implementation for this method, I could add it to the package if it is still needed/desired

[romk1n]

That would be great.