Add initial support for bucket lifecycle (#7563)

This PR is based off @sinhaashish's PR for object lifecycle
management, which includes support only for,
- Expiration of object
- Filter using object prefix (_not_ object tags)

N B the code for actual expiration of objects will be included in a
subsequent PR.

pkg/lifecycle/filter.go

@@ -0,0 +1,32 @@
+/*
+ * MinIO Cloud Storage, (C) 2019 MinIO, Inc.
+ *
+ * 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 lifecycle
+
+import "encoding/xml"
+
+// Filter - a filter for a lifecycle configuration Rule.
+type Filter struct {
+	XMLName xml.Name `xml:"Filter"`
+	And     And      `xml:"And,omitempty"`
+	Prefix  string   `xml:"Prefix"`
+	Tag     Tag      `xml:"Tag,omitempty"`
+}
+
+// Validate - validates the filter element
+func (f Filter) Validate() error {
+	return nil
+}