Implement SFTP server for file transfer over SSH protocol

[TranceLove]

A number of users had raised this, so I'm putting it up here anyway. Wish there will be enough interest and investments to throw in a bounty for kind-hearted devs to work on this.

Is your feature request related to a problem? Please describe. FTP server is considered obsolete and insecure for file transfers in 2022, even with SSL enabled. With modern devices' (even if it's a mobile phone) computation power it should not be difficult to handle file transmissions over SSH.

Describe the solution you'd like SFTP server replacing current Apache ftpserver implementation inside Amaze

Describe alternatives you've considered To reach the security we wish, just tell everyone to continue using FTP server with SSL enabled, and accept the fact that possibly only Filezilla can connect to it without problem.

Additional context Some considerations to push this through:

• There is already some code showing setting up a SFTP server already in the code tree, inside com.amaze.filemanager.filesystem.ssh.AbstractSftpServerTest
• Fixed key or generated key for SSH server?
• Hardest part maybe is to write a FileSystem implementation, especially with Android's troublesome DocumentFile implementation;
• Would MANAGE_ALL_FILES permission waive such troubles for us? But how about Androids that doesn't accept this permission?
• Will this implementation replacing, or just as an option aside the current FTP server?

[TranceLove]

Adding one more consideration: Apache SSHD uses Java File API that are available only from Android 8.0. While technically feasible, I don't see any good reason to backport everything to old File API which is too expensive, inefficient and error-prone.