chore(smithy-client): export common interfaces to remain backwards compatibility

Author: AllanZhengYP

packages/smithy-client/src/index.ts

@@ -9,3 +9,4 @@ export * from "./ser-utils";
 export * from "./date-utils";
 export * from "./split-every";
 export * from "./constants";
+export { DocumentType, SdkError, SmithyException } from "@aws-sdk/types";