Add example server that passes the test fixtures

Signed-off-by: Jake Sanders <i@am.so-aweso.me>

example/example.go

@@ -1 +1,61 @@
+// package example contains a self-contained example of a webhook that passes the cert-manager
+// DNS conformance tests
 package example
+
+import (
+	"sync"
+
+	"github.com/jetstack/cert-manager/pkg/acme/webhook"
+	acme "github.com/jetstack/cert-manager/pkg/acme/webhook/apis/acme/v1alpha1"
+	"github.com/miekg/dns"
+	"k8s.io/client-go/rest"
+)
+
+type exampleSolver struct {
+	name       string
+	server     *dns.Server
+	txtRecords map[string]string
+	sync.RWMutex
+}
+
+func (e exampleSolver) Name() string {
+	return e.name
+}
+
+func (e exampleSolver) Present(ch *acme.ChallengeRequest) error {
+	e.Lock()
+	e.txtRecords[ch.ResolvedFQDN] = ch.Key
+	e.Unlock()
+	return nil
+}
+
+func (e exampleSolver) CleanUp(ch *acme.ChallengeRequest) error {
+	e.Lock()
+	delete(e.txtRecords, ch.ResolvedFQDN)
+	e.Unlock()
+	return nil
+}
+
+func (e exampleSolver) Initialize(kubeClientConfig *rest.Config, stopCh <-chan struct{}) error {
+	go func(done <-chan struct{}) {
+		<-done
+		e.server.Shutdown()
+	}(stopCh)
+	go func() {
+		e.server.ListenAndServe()
+	}()
+	return nil
+}
+
+func New(port string) webhook.Solver {
+	e := &exampleSolver{
+		name:       "example",
+		txtRecords: make(map[string]string),
+	}
+	e.server = &dns.Server{
+		Addr:    ":" + port,
+		Net:     "udp",
+		Handler: dns.HandlerFunc(e.handleDNSRequest),
+	}
+	return e
+}