TUN-8640: Refactor ICMPRouter to support new ICMPResponders

A new ICMPResponder interface is introduced to provide different
implementations of how the ICMP flows should return to the QUIC
connection muxer.

Improves usages of netip.AddrPort to leverage the embedded zone
field for IPv6 addresses.

Closes TUN-8640

packet/encoder.go

@@ -1,6 +1,8 @@
 package packet
 
-import "github.com/google/gopacket"
+import (
+	"github.com/google/gopacket"
+)
 
 var (
 	serializeOpts = gopacket.SerializeOptions{
@@ -25,7 +27,7 @@ func NewEncoder() *Encoder {
 	}
 }
 
-func (e Encoder) Encode(packet Packet) (RawPacket, error) {
+func (e *Encoder) Encode(packet Packet) (RawPacket, error) {
 	encodedLayers, err := packet.EncodeLayers()
 	if err != nil {
 		return RawPacket{}, err