Add public-ipv4/public-ipv6 config options for manual IP override

On some servers ifconfig.co is unreachable (e.g. Hetzner, AdGuard DNS
blocklists), causing 'mtg doctor' SNI-DNS check and 'mtg access' link
generation to fail. New config options allow specifying public IPs
manually, with automatic detection as fallback.

Fixes #405

internal/cli/access.go

@@ -58,6 +58,9 @@ func (a *Access) Run(cli *CLI, version string) error {
 
 	wg.Go(func() {
 		ip := a.PublicIPv4
+		if ip == nil {
+			ip = conf.PublicIPv4.Get(nil)
+		}
 		if ip == nil {
 			ip = getIP(ntw, "tcp4")
 		}
@@ -70,6 +73,9 @@ func (a *Access) Run(cli *CLI, version string) error {
 	})
 	wg.Go(func() {
 		ip := a.PublicIPv6
+		if ip == nil {
+			ip = conf.PublicIPv6.Get(nil)
+		}
 		if ip == nil {
 			ip = getIP(ntw, "tcp6")
 		}