Add SFTP support to agent

## Please describe what you need help with:

Hello,
I'm using containerssh in kubernetes. Is it possible to use sshd in the container for handling sftp? (instead of sftp-server). I'm following up on #240, and I would like to use it's ChrootDirectory feature, but I can't get it working.

## Have you read the [troubleshooting guide](https://containerssh.io/reference/troubleshooting/)?

Knowing this information helps us avoid unnecessary questions.

## What have you tried to resolve this problem?

I tried setting
```
subsystems:
sftp: /sftp.sh
```
where sftp.sh calls `exec /usr/sbin/sshd -D`, but i guess it's not that simple, I don't fully understand how sftp works with containerssh.