exportLogs should return Future<void> instead of void

[PhenixZeller]

Currently when you use FlutterLogs.exportLogs(); in an async function, you can´t await it, because it is not a Future.

https://github.com/umair13adil/flutter_logs/blob/174350f1039dfeca7e175fe17b2922fb70e0660b/lib/flutter_logs.dart#L301

[umair13adil]

@PhenixZeller Noted, this will be done in the next release of the plugin.

[MrCsabaToth]

In the pull request I converted all static void async to return Future<void>