Add support for custom scripts

[aude]

This change adds support for putting custom scripts in /etc/sftp.d with the sftpConfig.customScripts Helm value.

Upstream atmoz/sftp looks in /etc/sftp.d for executables and runs them.

[aude]

Have been using the following script:

  ...
  customScripts:
    writable-homedirs.sh: |
      #!/bin/sh

      # make homedirs writable
      for homedir in /home/*; do
        if [ -d "$homedir" ]; then
          username=$(basename "$homedir")
          chown "$(id -u "$username"):$(id -g "$username")" "$homedir"
        fi
      done

      # chroot users to /home, not /home/<user>
      sed -E -i 's|^(ChrootDirectory\s+).*|\1/home|' '/etc/ssh/sshd_config'