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.

Start by tracing SftpClient.UploadFile and the internal UploadFile implementation, then compare it with the SftpStream approach described in the issue. Determine how interrupted transfers should resume while preserving existing upload behavior and throughput. Done means the option is clearly defined and resumable uploads work reliably.