Fix naming issue and implement IAsyncDisposable

[Poker-sang]

Dowloader's api design is handy and makes sense! But there are some methods with a Task return value, named without using the Async suffix, which confuses me when I use them. The Async suffix is also used in some void methods, such as DowloadService.CancelAsync.

Also I would like DowloadService to use DisposeAsync instead of Clear so that the await using syntax can be used.

Thank you!

[bezzad]

The Downloader release for all .Net versions such as .Net Framework 4.6. So the IAsyncDisposable is just supported in .Net Core and can't be used now. But, in the next version, I would remove support old versions from the Downloader and release it just for .Net 6 and 8.