Add DTLS CID client example

Adds a DTLS Connection ID client example. Two different "connections"
are used with the second resuming using the state from the first,
allowing it to bypass performing a second DTLS handshake.

Signed-off-by: Daniel Mangum <georgedanielmangum@gmail.com>

examples/dtls/cid/client/main.go

@@ -0,0 +1,77 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net"
+
+	piondtls "github.com/pion/dtls/v2"
+	"github.com/plgd-dev/go-coap/v3/dtls"
+)
+
+func main() {
+	conf := &piondtls.Config{
+		PSK: func(hint []byte) ([]byte, error) {
+			fmt.Printf("Server's hint: %s \n", hint)
+			return []byte{0xAB, 0xC1, 0x23}, nil
+		},
+		PSKIdentityHint:       []byte("Pion DTLS Client"),
+		CipherSuites:          []piondtls.CipherSuiteID{piondtls.TLS_PSK_WITH_AES_128_CCM_8},
+		ConnectionIDGenerator: piondtls.OnlySendCIDGenerator(),
+	}
+	raddr, err := net.ResolveUDPAddr("udp", "127.0.0.1:5688")
+	if err != nil {
+		log.Fatalf("Error resolving UDP address: %v", err)
+	}
+
+	// Setup first UDP listener.
+	udpconn, err := net.ListenUDP("udp", nil)
+	if err != nil {
+		log.Fatalf("Error establishing UDP listener: %v", err)
+	}
+
+	// Create DTLS client on UDP listener.
+	client, err := piondtls.Client(udpconn, raddr, conf)
+	if err != nil {
+		log.Fatalf("Error establishing DTLS client: %v", err)
+	}
+	co := dtls.Client(client)
+	resp, err := co.Get(context.Background(), "/a")
+	if err != nil {
+		log.Fatalf("Error performing request: %v", err)
+	}
+	log.Printf("Response payload: %+v", resp)
+	resp, err = co.Get(context.Background(), "/b")
+	if err != nil {
+		log.Fatalf("Error performing request: %v", err)
+	}
+	log.Printf("Response payload: %+v", resp)
+
+	// Export state to resume connection from another address.
+	state := client.ConnectionState()
+
+	// Setup second UDP listener on a different address.
+	udpconn, err = net.ListenUDP("udp", nil)
+	if err != nil {
+		log.Fatalf("Error establishing UDP listener: %v", err)
+	}
+
+	// Resume connection on new address with previous state.
+	client, err = piondtls.Resume(&state, udpconn, raddr, conf)
+	if err != nil {
+		log.Fatalf("Error resuming DTLS connection: %v", err)
+	}
+	co = dtls.Client(client)
+	// Requests can be performed without performing a second handshake.
+	resp, err = co.Get(context.Background(), "/a")
+	if err != nil {
+		log.Fatalf("Error performing request: %v", err)
+	}
+	log.Printf("Response payload: %+v", resp)
+	resp, err = co.Get(context.Background(), "/b")
+	if err != nil {
+		log.Fatalf("Error performing request: %v", err)
+	}
+	log.Printf("Response payload: %+v", resp)
+}