Add ObjectTagging Support (#8754)

This PR adds support for AWS S3 ObjectTagging API as explained here
https://docs.aws.amazon.com/AmazonS3/latest/dev/object-tagging.html

pkg/tagging/tag.go

@@ -0,0 +1,62 @@
+/*
+ * MinIO Cloud Storage, (C) 2020 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 tagging
+
+import (
+	"encoding/xml"
+	"unicode/utf8"
+)
+
+// Tag - single tag
+type Tag struct {
+	XMLName xml.Name `xml:"Tag"`
+	Key     string   `xml:"Key"`
+	Value   string   `xml:"Value"`
+}
+
+// Validate - validates the tag element
+func (t Tag) Validate() error {
+	if err := t.validateKey(); err != nil {
+		return err
+	}
+	if err := t.validateValue(); err != nil {
+		return err
+	}
+	return nil
+}
+
+// validateKey - checks if key is valid or not.
+func (t Tag) validateKey() error {
+	// cannot be longer than maxTagKeyLength characters
+	if utf8.RuneCountInString(t.Key) > maxTagKeyLength {
+		return ErrInvalidTagKey
+	}
+	// cannot be empty
+	if len(t.Key) == 0 {
+		return ErrInvalidTagKey
+	}
+	return nil
+}
+
+// validateValue - checks if value is valid or not.
+func (t Tag) validateValue() error {
+	// cannot be longer than maxTagValueLength characters
+	if utf8.RuneCountInString(t.Value) > maxTagValueLength {
+		return ErrInvalidTagValue
+	}
+	return nil
+}