fix(cloud_firestore): Export enum LoadBundleTaskState from Platform Interface package.

[jarrodonlo]

Description

FlutterFire's example at https://firebase.flutter.dev/docs/firestore/usage/#data-bundles uses the enum LoadBundleTaskState (snippet below).
However, the package currently does not export, requiring the developer to import from source.

// Use .stream API to expose a stream which listens for LoadBundleTaskSnapshot events.
task.stream.listen((taskStateProgress) {
  if(taskStateProgress.taskState == LoadBundleTaskState.success){
    //bundle is loaded into app cache!
  }
});

Related Issues

N/A

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[russellwheatley]

Hey @jarrodonlo, do you mind signing the CLA, please? Then we can get this merged in. Thanks.

[russellwheatley]

LGTM