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/manifests/crds/coredns.io_forwards.yaml

@@ -0,0 +1,66 @@
+
+---
+apiVersion: apiextensions.k8s.io/v1
+kind: CustomResourceDefinition
+metadata:
+  annotations:
+    controller-gen.kubebuilder.io/version: v0.4.1
+  creationTimestamp: null
+  name: forwards.coredns.io
+spec:
+  group: coredns.io
+  names:
+    kind: Forward
+    listKind: ForwardList
+    plural: forwards
+    singular: forward
+  scope: Namespaced
+  versions:
+  - additionalPrinterColumns:
+    - jsonPath: .spec.from
+      name: From
+      type: string
+    - jsonPath: .spec.to
+      name: To
+      type: string
+    name: v1alpha1
+    schema:
+      openAPIV3Schema:
+        description: Forward represents a zone that should have its DNS requests forwarded
+          to an upstream DNS server within CoreDNS
+        properties:
+          apiVersion:
+            description: 'APIVersion defines the versioned schema of this representation
+              of an object. Servers should convert recognized schemas to the latest
+              internal value, and may reject unrecognized values. More info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#resources'
+            type: string
+          kind:
+            description: 'Kind is a string value representing the REST resource this
+              object represents. Servers may infer this from the endpoint the client
+              submits requests to. Cannot be updated. In CamelCase. More info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#types-kinds'
+            type: string
+          metadata:
+            type: object
+          spec:
+            description: ForwardSpec represents the spec of a Forward
+            properties:
+              from:
+                type: string
+              to:
+                items:
+                  type: string
+                type: array
+            type: object
+          status:
+            description: ForwardStatus represents the status of a Forward
+            type: object
+        type: object
+    served: true
+    storage: true
+    subresources: {}
+status:
+  acceptedNames:
+    kind: ""
+    plural: ""
+  conditions: []
+  storedVersions: []