Offer waiting for a specific download task

[MarcelGarus]

It would be great if the end-developer could just wait for a certain task, perhaps by calling something in the likes of await FlutterDownloader.waitForTask(taskId);

It wouldn't be too hard to implement – especially after #252 is implemented, there's already a stream of updates coming from the background isolate, so the function could simply create a Completer and complete that when the right update event comes through.

[SpanishOnion]

I think this API is necessary, and here we need a state value that can represent the queuing, and we can switch the state to paused.

[MarcelGarus]

I'm currently implementing the new API mentioned in this other issue. In this new API, DownloadTasks are powerful objects, so these will get a new wait method that waits for the download to complete or fail or getting canceled.