use plugin registry and prepare for conversion (#1003)

* use plugin registry and prepare for conersion

* Register plugins explicitly to a registry

* check interface impl instead of struc var

* setup plugins at top level

* treat plugin type combinations

* pass registry as arg of V1alpha1ToInternal

* move registry yet another level up

* check interface type separately

pkg/framework/plugins/pluginbuilder/pluginbuilder.go

@@ -0,0 +1,48 @@
+/*
+Copyright 2022 The Kubernetes Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+    http://www.apache.org/licenses/LICENSE-2.0
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package pluginbuilder
+
+import (
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/klog/v2"
+	"sigs.k8s.io/descheduler/pkg/framework"
+)
+
+var PluginRegistry Registry
+
+type PluginBuilderAndArgsInstance struct {
+	PluginBuilder PluginBuilder
+	// Just an example instance of this PluginArg so we can avoid having
+	// to deal with reflect Types
+	PluginArgInstance runtime.Object
+}
+
+type PluginBuilder = func(args runtime.Object, handle framework.Handle) (framework.Plugin, error)
+
+type Registry = map[string]PluginBuilderAndArgsInstance
+
+func NewRegistry() Registry {
+	return Registry{}
+}
+
+func Register(name string, builderFunc PluginBuilder, exampleArg runtime.Object, registry Registry) {
+	if _, ok := registry[name]; ok {
+		klog.V(10).InfoS("Plugin already registered", "plugin", name)
+	} else {
+		registry[name] = PluginBuilderAndArgsInstance{
+			PluginBuilder:     builderFunc,
+			PluginArgInstance: exampleArg,
+		}
+	}
+}