v0.2.0 of sia_storage

Dart bindings for the Sia Storage SDK. Upload, download, and pin objects on the Sia network from Flutter and pure Dart applications. Changelog excerpt: ### Breaking Changes #### Rename uploadStream/downloadStreamto upload/download The stream-based helpers on Sdkare renamed to match the underlying SDK: uploadStreamis now uploadand downloadStreamis now download. The raw callback/StreamSinkbindings they wrapped are no longer exposed on the public Sdktype. Migrate by dropping the Streamsuffix: ``` `// before final upload = sdk.uploadStream(object: obj, source: file.openRead()); final download = sdk.downloadStream(object:[...]