Fix linkerd dg endpoints to work with IPv6 (#12541)

```
# BEFORE
$ bin/linkerd dg endpoints family-server.default.svc.cluster.local:8080
NAMESPACE   IP        PORT   POD                              SERVICE
default     0.0.0.0   8080   family-server-7cf95b6b89-x6cb9   family-server.default

# AFTER
$ bin/linkerd dg endpoints family-server.default.svc.cluster.local:8080
NAMESPACE   IP                      PORT   POD                              SERVICE
default     fd00:10:244::5          8080   family-server-7cf95b6b89-x6cb9   family-server.default
```

cli/cmd/endpoints.go

@@ -3,11 +3,9 @@ package cmd
 import (
 	"bytes"
 	"context"
-	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"fmt"
-	"net"
 	"os"
 	"sort"
 	"strings"
@@ -17,6 +15,7 @@ import (
 	destinationPb "github.com/linkerd/linkerd2-proxy-api/go/destination"
 	netPb "github.com/linkerd/linkerd2-proxy-api/go/net"
 	"github.com/linkerd/linkerd2/controller/api/destination"
+	"github.com/linkerd/linkerd2/pkg/addr"
 	pkgcmd "github.com/linkerd/linkerd2/pkg/cmd"
 	"github.com/linkerd/linkerd2/pkg/k8s"
 	log "github.com/sirupsen/logrus"
@@ -218,10 +217,11 @@ func requestEndpointsFromAPI(client destinationPb.DestinationClient, token strin
 }
 
 func getIP(tcpAddr *netPb.TcpAddress) string {
-	ip := tcpAddr.GetIp().GetIpv4()
-	b := make([]byte, 4)
-	binary.BigEndian.PutUint32(b, ip)
-	return net.IP(b).String()
+	ip := addr.FromProxyAPI(tcpAddr.GetIp())
+	if ip == nil {
+		return ""
+	}
+	return addr.PublicIPToString(ip)
 }
 
 func renderEndpoints(endpoints endpointsInfo, options *endpointsOptions) string {

pkg/addr/addr.go

@@ -46,7 +46,7 @@ func PublicIPToString(ip *l5dNetPb.IPAddress) string {
 
 // ProxyAddressToString formats a Proxy API TCPAddress as a string.
 func ProxyAddressToString(addr *pb.TcpAddress) string {
-	vizIP := proxyToVizIPAddr(addr.GetIp())
+	vizIP := FromProxyAPI(addr.GetIp())
 	if vizIP == nil {
 		return ""
 	}
@@ -90,21 +90,23 @@ func ParsePublicIP(ip string) (*l5dNetPb.IPAddress, error) {
 	if err != nil {
 		return nil, err
 	}
-	return proxyToVizIPAddr(addr), nil
+	return FromProxyAPI(addr), nil
 }
 
 // NetToPublic converts a Proxy API TCPAddress to a Viz API
 // TCPAddress.
 func NetToPublic(net *pb.TcpAddress) *l5dNetPb.TcpAddress {
-	ip := proxyToVizIPAddr(net.GetIp())
+	ip := FromProxyAPI(net.GetIp())
 
 	return &l5dNetPb.TcpAddress{
 		Ip:   ip,
 		Port: net.GetPort(),
 	}
 }
 
-func proxyToVizIPAddr(net *pb.IPAddress) *l5dNetPb.IPAddress {
+// FromProxyAPI casts an IPAddress from the linkerd2-proxy-api.go.net package
+// to the linkerd2.controller.gen.common.net package
+func FromProxyAPI(net *pb.IPAddress) *l5dNetPb.IPAddress {
 	switch ip := net.GetIp().(type) {
 	case *pb.IPAddress_Ipv6:
 		return &l5dNetPb.IPAddress{