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