chore(netlink): separate linux only and OS independent code

- Move `Addr` and its `String` method to `types.go`
- Move `IsWireguardSupported` to `wireguard.go` to have `family.go` OS independant
- Remove dependency on vishvananda/netlink in `ipv6.go`
- Move `Link` to `types.go`
- Move `Route` to `types.go`
- Move `Rule` and its `String` method to `types.go`

internal/netlink/wireguard_test.go

@@ -0,0 +1,21 @@
+package netlink
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_NetLink_IsWireguardSupported(t *testing.T) {
+	t.Skip() // TODO unskip once the data race problem with netlink.GenlFamilyList() is fixed
+
+	t.Parallel()
+	netLink := &NetLink{}
+	ok, err := netLink.IsWireguardSupported()
+	require.NoError(t, err)
+	if ok { // cannot assert since this depends on kernel
+		t.Log("wireguard is supported")
+	} else {
+		t.Log("wireguard is not supported")
+	}
+}