fix(@cubejs-client/core): make progressCallback optional (#497) Thanks to @hassankhan!

Author: hassankhan

packages/cubejs-client-core/dist/index.d.ts

@@ -12,7 +12,7 @@ export type CubeJSApiOptions = {
 export type LoadMethodOptions = {
   mutexKey?: string;
   mutexObj?: {};
-  progressCallback(result: ProgressResult): void;
+  progressCallback?(result: ProgressResult): void;
   subscribe?: boolean;
 };