Add Opensearch field type for flattened mapping

opensearch/time_series.go

@@ -194,6 +194,8 @@ func structMappingProperties(structType reflect.Type) (map[string]MappingPropert
 		case "object":
 			fieldMapping.Type = "object"
 			fieldMapping.Enabled = false
+		case "flattened":
+			fieldMapping.Type = "flat_object"
 		case "-":
 			// do not include in mapping
 			skip = true