Set maximum packet size for sftp upload

### Discussed in https://github.com/hierynomus/sshj/discussions/798

Hello, may I ask if there is any possibility to set maximum packet size for sftp upload? We are having problems with bad network connection in some places, where we need to upload some bigger files and from what I found in the code, packet size is determined from MTU (max transfer unit) on the device. Is there any chance how to set it manually for net.schmizz.sshj.sftp.SftpClient? . Goal is to make packets smaller than MTU.