[os-tailscale] WebUI Support for Tailscale Peer Relay Server functionality

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Is your feature request related to a problem? Please describe.

Could it be possible to include Tailscale Relay Server setting up under OPNsense UI Tailscale configuration?

Relay server functionality is already included in the Tailscale plugin binary:

Currently in order to have Relay server in OPNsense, the following command should executed manually in a shell:
tailscale set --relay-server-port=40000

Describe the solution you'd like
What I am suggesting?:

Adding the following additional visual elements under OPNSense Tailscale WebUI configuration screen (/ui/tailscale/settings)

• Use as Relay server: Y/N.
• Relay server port. (i.e. 40000 the default one)

Additionally, it could make sense to create automatically a firewall rule to allow Relay server port traffic.

Describe alternatives you've considered
I have created a custom boot script:

cat << 'EOF' > /usr/local/etc/rc.syshook.d/start/99-tailscale-relay-port.sh
#!/bin/sh
/usr/local/bin/tailscale set --relay-server-port=40000
EOF

Additional context

• https://tailscale.com/docs/features/peer-relay
• https://tailscale.com/docs/features/peer-relay

Start at the Tailscale settings page entry point, /ui/tailscale/settings, and review how existing configuration values are stored and applied. Trace the current plugin command handling, then determine how the relay-server-port setting and enable control should be represented; done means the WebUI configures the relay server and the requested port, with firewall handling clarified.