feat(connections, nat): add UDP portmapping/pinhole for QUIC (fixes #7403) (#10171)

Fixes #7403.

Tested by enabling UPnP on the router, and checking on the router page
that the external ports of the UDP mappings match what is shown in the
logs and the internal ports matching the QUIC listening port.

lib/connections/util.go

@@ -12,6 +12,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/syncthing/syncthing/lib/nat"
 	"github.com/syncthing/syncthing/lib/osutil"
 )
 
@@ -130,3 +131,27 @@ func maybeReplacePort(uri *url.URL, laddr net.Addr) *url.URL {
 	uriCopy.Host = net.JoinHostPort(host, lportStr)
 	return &uriCopy
 }
+
+func portMappingURIs(mapping *nat.Mapping, listener_uri url.URL) []*url.URL {
+	var uris []*url.URL
+	if mapping != nil {
+		addrs := mapping.ExternalAddresses()
+		for _, addr := range addrs {
+			uri := listener_uri
+			// Does net.JoinHostPort internally
+			uri.Host = addr.String()
+			uris = append(uris, &uri)
+
+			// For every address with a specified IP, add one without an IP,
+			// just in case the specified IP is still internal (router behind DMZ).
+			if len(addr.IP) != 0 && !addr.IP.IsUnspecified() {
+				zeroUri := listener_uri
+				addr.IP = nil
+				zeroUri.Host = addr.String()
+				uris = append(uris, &zeroUri)
+			}
+		}
+	}
+
+	return uris
+}