Initial implementation of ForwardCRD plugin (#4512)

* Add forwardcrd plugin README.md

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* Create forwardcrd plugin

- Place forwardcrd before forward plugin in plugin list. This will avoid
forward from preventing the forwardcrd plugin from handling any queries
in the case of having a default upstream forwarder in a server block (as
is the case in the default kubernetes Corefile).

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* Add Forward CRD

Signed-off-by: Christian Ang <angc@vmware.com>

* Add NewWithConfig to forward plugin

- allows external packages to instanciate forward plugins

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* ForwardCRD plugin handles requests for Forward CRs

- add a Kubernetes controller that can read Forward CRs
- instances of the forward plugin are created based on Forward CRs from
the Kubernetes controller
- DNS requests are handled by calling matching Forward plugin instances
based on zone name
- Defaults to the kube-system namespace to align with Corefile RBAC

Signed-off-by: Christian Ang <angc@vmware.com>

Use klog v2 in forwardcrd plugin

* Refactor forward setup to use NewWithConfig

Co-authored-by: Christian Ang <angc@vmware.com>

Signed-off-by: Edwin Xie <exie@vmware.com>

* Use ParseInt instead of Atoi

- to ensure that the bitsize is 32 for later casting to uint32

Signed-off-by: Christian Ang <angc@vmware.com>

* Add @christianang to CODEOWNERS for forwardcrd

Signed-off-by: Christian Ang <angc@vmware.com>

Co-authored-by: Edwin Xie <exie@vmware.com>

plugin/forwardcrd/fakes_test.go

@@ -0,0 +1,86 @@
+package forwardcrd
+
+import (
+	"context"
+	"sync"
+
+	"github.com/coredns/coredns/plugin/forward"
+
+	"github.com/miekg/dns"
+)
+
+type TestPluginHandler struct {
+	mutex               sync.Mutex
+	ReceivedConfig      forward.ForwardConfig
+	onStartupCallCount  int
+	onShutdownCallCount int
+}
+
+func (t *TestPluginHandler) ServeDNS(context.Context, dns.ResponseWriter, *dns.Msg) (int, error) {
+	return 0, nil
+}
+
+func (t *TestPluginHandler) Name() string { return "" }
+
+func (t *TestPluginHandler) OnStartup() error {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	t.onStartupCallCount++
+	return nil
+}
+
+func (t *TestPluginHandler) OnShutdown() error {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	t.onShutdownCallCount++
+	return nil
+}
+
+func (t *TestPluginHandler) OnStartupCallCount() int {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	return t.onStartupCallCount
+}
+
+func (t *TestPluginHandler) OnShutdownCallCount() int {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	return t.onShutdownCallCount
+}
+
+type TestPluginInstancer struct {
+	mutex              sync.Mutex
+	testPluginHandlers []*TestPluginHandler
+}
+
+func (t *TestPluginInstancer) NewWithConfig(config forward.ForwardConfig) (lifecyclePluginHandler, error) {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+
+	testPluginHandler := &TestPluginHandler{
+		ReceivedConfig: config,
+	}
+	t.testPluginHandlers = append(t.testPluginHandlers, testPluginHandler)
+	return testPluginHandler, nil
+}
+
+func (t *TestPluginInstancer) NewWithConfigArgsForCall(index int) *TestPluginHandler {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+
+	return t.testPluginHandlers[index]
+}
+
+func (t *TestPluginInstancer) NewWithConfigCallCount() int {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+
+	return len(t.testPluginHandlers)
+}
+
+type TestController struct {
+}
+
+func (t *TestController) Run(threads int) {}
+func (t *TestController) HasSynced() bool { return true }
+func (t *TestController) Stop() error     { return nil }