Add UploadFileAsync and DownloadFileAsync methods

[sdt-ndelarosa]

Adds TAP methods for UploadFile and DownloadFile to ISftpClient.

Other Solutions

While there are other options to define both the UploadFileAsync and DownloadFileAsync methods, such as the extension methods approach and the previously proposed pull request, #1515 , this pull request has the following benefits over the other solutions:

• First class async/await support for uploading and downloading
  • Users no longer have to rely on implementing their own extension methods
• Cleaner interop with the sync versions of the UploadFile and DownloadFile methods
  • Method signatures have a closer match
• Upload and downloads now work with relative paths instead of just absolute paths
• Deeper async/await call chain, as compared to BeginUploadFile, which pretty much runs the sync UploadFile method, but just in a different thread

[sdt-ndelarosa]

actions don't seem to be triggering, gonna try closing and reopening

[Rob-Hague]

Hi, I will make time to review most likely at some point next week. FYI for first time contributors the CI needs to be run manually (some kind of github security measure)

[Rob-Hague]

Looks good. I think it could do with testing cancellation - i.e. that it throws OperationCanceledException when the token is cancelled.

It would probably not be very reliable to test cancelling during a file download (at least not in an integration test), but we could at least have a test for when a cancelled token is supplied