Continue Interrupted file uploads

Hi,

I am in a situation where I am uploading large files via SFTP , and sometimes this gets interrupted. The next time I try I want to continue where it stopped. The provided SftpClient.UploadFile functionality did not have that option, so I wrote it using an SftpStream to write to. However the throughput of that was something like 4 to 5 times lower than a call to uploadfile.

So I ended up forking, and added a parameter "continueAbortedTransfer", to the
(Internal)UploadFile function. And a few lines of code at the start of the function to set
the stream positions correctly if the user wants to continue where it stopped the last time.

I have some questions:

1. Is there an interest in this option, and should I try to prepare a pull request?
2. Is the best way to add this? I know very little about the internals of this library,
   this was the first location that I found and it is just a couple of lines of code, but
   it could well be that there are better ways to add options like this, or that I broke
   some other code in the process.

Beginne damit, SftpClient.UploadFile und die interne UploadFile-Implementierung nachzuverfolgen, und vergleiche sie anschließend mit dem im Issue beschriebenen SftpStream-Ansatz. Ermittle, wie unterbrochene Übertragungen fortgesetzt werden sollen, wobei das bestehende Upload-Verhalten und der Durchsatz erhalten bleiben. Als abgeschlossen gilt die Aufgabe, wenn die Option eindeutig definiert ist und fortsetzbare Uploads zuverlässig funktionieren.