Split plugin framework into its own package

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

pkg/plugin/framework/interface.go

@@ -0,0 +1,28 @@
+/*
+Copyright 2017, 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 plugin "github.com/hashicorp/go-plugin"
+
+// Interface represents a Velero plugin.
+type Interface interface {
+	plugin.Plugin
+
+	// names returns a list of all the registered implementations for this plugin (such as "pod" and "pvc" for
+	// BackupItemAction).
+	names() []string
+}