Improve Security of Remote Support Sessions

[gsanchietti]

Enhance the security of remote support sessions by implementing the following changes:

• Limit session longevity by stopping support sessions after 24 hours. 24 hours is a soft limit that can be manually increased by operators up to 7 days. The limit of 7 days is enforced also on the VPN server side.
• Show the session expiration time inside the UI.
• Ensure SSH is only accessible via the support VPN to prevent unauthorized direct access.
• Evaluate and possibly enforce restrictions to allow access to the web interface with the support user only from a specified list of IP addresses.

Discussion:

• https://mattermost.nethesis.it/nethesis/pl/skr8ar3wkjft3ewxqdynzfar9a
• https://mattermost.nethesis.it/nethesis/pl/fgegji66rffy7jqyj5m9kxuzqw

[DavidePrincipi]

Ensure SSH is only accessible via the support VPN to prevent unauthorized direct access.

This requirement is already enforced in .ssh/authorized_keys with from=127.0.0.1: https://github.com/NethServer/ns8-core/blob/6cc50bcf3c64357b88025a6a48ea6d2c73c219f3/core/imageroot/var/lib/nethserver/node/bin/support-sshkeyctl#L19