Split plugin framework into its own package

Signed-off-by: Carlisia <carlisiac@vmware.com>

pkg/plugin/framework/plugin_kinds.go

@@ -0,0 +1,56 @@
+/*
+Copyright 2018, 2019 the Velero contributors.
+
+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 framework
+
+import (
+	"k8s.io/apimachinery/pkg/util/sets"
+)
+
+// PluginKind is a type alias for a string that describes
+// the kind of a Velero-supported plugin.
+type PluginKind string
+
+// String returns the string for k.
+func (k PluginKind) String() string {
+	return string(k)
+}
+
+const (
+	// PluginKindObjectStore represents an object store plugin.
+	PluginKindObjectStore PluginKind = "ObjectStore"
+
+	// PluginKindBlockStore represents a block store plugin.
+	PluginKindBlockStore PluginKind = "BlockStore"
+
+	// PluginKindBackupItemAction represents a backup item action plugin.
+	PluginKindBackupItemAction PluginKind = "BackupItemAction"
+
+	// PluginKindRestoreItemAction represents a restore item action plugin.
+	PluginKindRestoreItemAction PluginKind = "RestoreItemAction"
+
+	// PluginKindPluginLister represents a plugin lister plugin.
+	PluginKindPluginLister PluginKind = "PluginLister"
+)
+
+// allPluginKinds contains all the valid plugin kinds that Velero supports, excluding PluginLister because that is not a
+// kind that a developer would ever need to implement (it's handled by Velero and the Velero plugin library code).
+var allPluginKinds = sets.NewString(
+	PluginKindObjectStore.String(),
+	PluginKindBlockStore.String(),
+	PluginKindBackupItemAction.String(),
+	PluginKindRestoreItemAction.String(),
+)