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.

Commencez par suivre SftpClient.UploadFile et l’implémentation interne de UploadFile, puis comparez-la avec l’approche SftpStream décrite dans l’issue. Déterminez comment les transferts interrompus doivent reprendre, tout en préservant le comportement existant des uploads et le débit. Le travail est terminé lorsque l’option est clairement définie et que les uploads reprenables fonctionnent de manière fiable.