Refactor watch pods to allow reusing watch wrapper (#470)

Currently shared/kubernetes/watch.go:FilteredWatch only watches pods.
This PR makes it reusable for other types of resources.
This is done in preparation for watching k8s events.

shared/kubernetes/watchEvent.go

@@ -0,0 +1,18 @@
+package kubernetes
+
+import (
+	"fmt"
+	corev1 "k8s.io/api/core/v1"
+	"k8s.io/apimachinery/pkg/watch"
+)
+
+type WatchEvent watch.Event
+
+func (we *WatchEvent) ToPod() (*corev1.Pod, error) {
+	pod, ok := we.Object.(*corev1.Pod)
+	if !ok {
+		return nil, fmt.Errorf("Invalid object type on pod event stream")
+	}
+
+	return pod, nil
+}