feat(ns-plug): add MTU test and configurable MTU field

[Tbaile]

Brief description

Currently, users experience connectivity issues due to incorrect MTU settings between NethSecurity firewalls and controllers, especially in low-quality network environments. Workarounds like manually setting MTU on the tunnel or using ifconfig are not persistent after VPN reconnections, and troubleshooting is difficult without clear visibility into MTU-related problems.

Proposed solution

• Integrate an MTU test into the ns-plug client so logs immediately highlight MTU issues, allowing easier diagnosis with a simple grep.
• Add a configurable MTU field to ns-plug's configuration, making MTU adjustments persistent and manageable by users directly from configuration—no manual intervention needed after VPN reconnects.

Alternative solutions

• Only expose MTU setting on either the controller or firewall (less flexible, may not resolve all scenarios).
• Use OpenVPN options (--tun-mtu, --fragment, --mssfix) for more dynamic adaptation, but requires additional patching and environment variable support in the VPN container/module.

Additional context

• Reference implementation ideas from

  nethsecurity/packages/ns-plug/files/ns-plug

  Lines 98 to 127 in b30a429

  	# Configuration received, setup the VPN
  	host=$(echo ${response} | jq -r .data.host)
  	port=$(echo ${response} | jq -r .data.port)
  	cert=$(echo ${response} | jq -r .data.cert)
  	key=$(echo ${response} | jq -r .data.key)
  	ca=$(echo ${response} | jq -r .data.ca)
  	cat <<EOF > ${CONFIG_FILE}
  	client
  	server-poll-timeout 5
  	nobind
  	float
  	explicit-exit-notify 1
  	remote ${host} ${port} udp
  	connect-retry-max 5
  	dev tun-nsplug
  	tls-client
  	script-security 2
  	route-up /usr/sbin/ns-controller-push-info
  	<ca>
  	${ca}
  	</ca>
  	<cert>
  	${cert}
  	</cert>
  	<key>
  	${key}
  	</key>
  	auth-nocache
  	verb 3
  	EOF

References

• Helpdesk ticket: https://helpdesk.nethesis.it/a/tickets/191784
• Mattermost Discussion: https://mattermost.nethesis.it/nethesis/pl/qpm7auwxtfn8jeh1i875mmm46o