feat(pmtud/tcp): support mixed IPv4 and IPv6 TCP servers

- Add default cloudflare and google tls ipv6 servers to default tcp servers
- update integration test to try against both ipv4 and ipv6 servers

internal/pmtud/ip/family.go

@@ -0,0 +1,27 @@
+package ip
+
+import (
+	"net/netip"
+	"slices"
+
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+)
+
+func GetFamilies(dsts []netip.AddrPort) (families []int) {
+	const maxFamilies = 2
+	families = make([]int, 0, maxFamilies)
+	for _, dst := range dsts {
+		family := GetFamily(dst)
+		if !slices.Contains(families, family) {
+			families = append(families, family)
+		}
+	}
+	return families
+}
+
+func GetFamily(dst netip.AddrPort) int {
+	if dst.Addr().Is4() {
+		return constants.AF_INET
+	}
+	return constants.AF_INET6
+}