Add support for resuming SFTP file upload/download

[zybexXL]

This adds resume capability to SftpClient.UploadFile() , SftpClient.DownloadFile(), as well as to the BeginUploadFile() and BeginDownloadFile() variants.

The API remains unchanged. Resume is triggered simply by changing the current Position of the input/output stream which is given as an argument to these functions:

• Setting a non-zero position on the input Stream for UploadFile() triggers an Append to the remote file. It's up to the caller to first check the current size of the remote file and set the local position accordingly!
• Setting a non-zero position on the output Stream for DownloadFile() triggers a resume of the download at the given position.

This also does NOT check if the server supports Resume, though all servers I've tested so far do support it.

[zybexXL]

I've rebased this PR, please review and consider merging.

[WojciechNagorski]

I checked this PR and it looks great!

I just want you to provide two integration tests for upload and download.
You can use similar mechanism, like in

SSH.NET/test/Renci.SshNet.IntegrationTests/ConnectivityTests.cs

Line 199 in 2eec748

public void Common_LossOfNetworkConnectivityDisconnectAndConnect()

bool vmNetworkConnectionDisabled = false;
SshConnectionRestorer disruptor = null;
try
{
    using (var client = new SftpClient(_connectionInfoFactory.Create()))
    {
        client.Connect();
        
        // <-- Start upload/download file

        disruptor = _sshConnectionDisruptor.BreakConnections();
        vmNetworkConnectionDisabled = true;

        WaitForConnectionInterruption(client);
        // disconnect while network connectivity is lost
        client.Disconnect();

        Assert.IsFalse(client.IsConnected);
        
        disruptor.RestoreConnections();
        vmNetworkConnectionDisabled = false;

        // connect when network connectivity is restored
        client.Connect();

        // <-- Resume download/upload file

        client.Dispose();
    }
}
finally
{
    if (vmNetworkConnectionDisabled)
    {
        disruptor.RestoreConnections();
    }
    disruptor?.Dispose();
}

[zybexXL]

@WojciechNagorski
This type of Resume test won't work as the test file may well finish upload/download before the Disruptor kicks in, especially if the test server is in localhost.
Better to upload a small file, then reconnect and append some more data, then check if the filesize is as expected. The download resume test can download the file in 2 chunks and check if it's correct. OK ?

[WojciechNagorski]

Sorry for delay. It's correct.