SFTPClient get methods do not allow to append the whole source to the destination

# What I want to do

Using the `SFTPClient` class, I would like to be able to download a source file from the SFTP server and **append all** of its content to the destination file.

# What the library allows to do

Using `SFTPClient` `get()` methods either :
- Download the source and overwrite the destination
- Download the source and append it to the destination with the specified `byteOffset`. But if `byteOffset` is set to `0`, the first case is occurring instead.

I think the issue comes from the `SFTPFileTransfer` `downloadFile` method :
```
final OutputStream os = adjusted.getOutputStream(byteOffset != 0);
```

# My workaround

For now, I am using the code below to perform what I want to do :
```
private CustomSSHClient mSshClient;
private SFTPClient mSftpClient;
...
try {
SFTPEngine engine = mSftpClient.getSFTPEngine();
SFTPFileTransfer fileTransfer = mSftpClient.getFileTransfer();

RemoteResourceInfo remoteResourceInfo = new RemoteResourceInfo(
engine.getPathHelper().getComponents(srcFilePath),
engine.stat(srcFilePath)
);

final LocalDestFile adjusted = new FileSystemFile(desFilePath).getTargetFile(remoteResourceInfo.getName());
final RemoteFile rf = engine.open(remoteResourceInfo.getPath());

final RemoteFile.ReadAheadRemoteFileInputStream rfis = rf.new ReadAheadRemoteFileInputStream(16, 0);
final OutputStream os = adjusted.getOutputStream(true);

new StreamCopier(rfis, os, mSshClient.getLoggerFactory())
.bufSize(engine.getSubsystem().getLocalMaxPacketSize())
.keepFlushing(false)
.listener(fileTransfer.getTransferListener().file(remoteResourceInfo.getName(), remoteResourceInfo.getAttributes().getSize()))
.copy();

rfis.close();
os.close();
rf.close();
} catch (IOException exception) {

}
...
public class CustomSSHClient extends SSHClient{
public LoggerFactory getLoggerFactory(){
return loggerFactory;
}
}
```