SftpClient: add IProgress to DownloadFileAsync and UploadFileAsync

[mus65]

fixes #1765

[mus65]

Build failure unrelated, opened #1772 .

[Rob-Hague]

I'd be inclined to go with IProgress<T> instead of Action for new/async APIs - Progress<T> provides some handling of the SynchronizationContext. And perhaps make T := struct DownloadFileProgressReport { long BytesDownloaded } or similar so that e.g. another property could be added later without much friction. WDYT?

Also do you want to do the same for UploadFileAsync as well? I guess it could be added to

Task UploadFileAsync(Stream input, string path, bool canOverride, CancellationToken cancellationToken = default);

since it hasn't made a release yet. All parameters after the first two could be made optional.

[mus65]

I'd be inclined to go with IProgress instead of Action for new/async APIs

Done. This required another overload for UploadFileAsync though. We can't make canOverride optional since that would make UploadAsync(stream, file) ambigous.