os-tailscale peer relay option

[hyperboly]

This PR adds peer relay option to os-tailscale.

Creates a field for peer relay port, if 0 peer relay is disabled.

Related: #5122

[hyperboly]

tailscale up won't accept --relay-server-port as an arg, only tailscale set. Working on adding tailscale set to the plugin.

[sheridans]

The --relay-server-port flag itself is defined in:
https://raw.githubusercontent.com/tailscale/tailscale/refs/heads/main/cmd/tailscale/cli/ssh.go

Specifically:

setf.StringVar(&setArgs.relayServerPort, "relay-server-port", "", "UDP port number (0 will pick a random unused port) for the relay server to bind to, on all interfaces, or empty string to disable relay server functionality")

Later in the same file, non-empty values get parsed an assigned:

if setArgs.relayServerPort != "" {
	uport, err := strconv.ParseUint(setArgs.relayServerPort, 10, 16)
	if err != nil {
		return fmt.Errorf("failed to set relay server port: %v", err)
	}
	maskedPrefs.Prefs.RelayServerPort = ptr.To(uint16(uport))
}

This would change the default expected behaviour of Tailscale:

• Required field, set to 0 by default to disable later in rc template. Tailscale uses a value of 0to use a random port and an empty string value to disable
• Prevents the user from specifying a port value of 0 to use a random port.

@fichtner unless I am wrong shouldn't be a required field, should be empty by default, allowed values 0-65535? Template logic would also need adjusting, and if supported flag for tailscale up args.