event: Set contentType as well under NotificationEvent. (#4003)

This is an enhancement change to to cater support all
the data fields present on the object. Currently
we only send a subset of data which object info
provides us.

It also helps us keep a full namespace mirror on
notification targets for efficient query.

cmd/bucket-notification-datatypes.go

@@ -136,11 +136,13 @@ type bucketMeta struct {
 
 // Notification event object metadata.
 type objectMeta struct {
-	Key       string `json:"key"`
-	Size      int64  `json:"size,omitempty"`
-	ETag      string `json:"eTag,omitempty"`
-	VersionID string `json:"versionId,omitempty"`
-	Sequencer string `json:"sequencer"`
+	Key         string            `json:"key"`
+	Size        int64             `json:"size,omitempty"`
+	ETag        string            `json:"eTag,omitempty"`
+	ContentType string            `json:"contentType:omitempty"`
+	UserDefined map[string]string `json:"userDefined:omitempty"`
+	VersionID   string            `json:"versionId,omitempty"`
+	Sequencer   string            `json:"sequencer"`
 }
 
 const (