Add compressed file index (#15247)

cmd/object-api-interface.go

@@ -78,6 +78,10 @@ type ObjectOptions struct {
 	WalkAscending bool // return Walk results in ascending order of versions
 
 	PrefixEnabledFn func(prefix string) bool // function which returns true if versioning is enabled on prefix
+
+	// IndexCB will return any index created but the compression.
+	// Object must have been read at this point.
+	IndexCB func() []byte
 }
 
 // ExpirationOptions represents object options for object expiration at objectLayer.