chore(toolkit): export source builder props

Author: mrgrain

packages/@aws-cdk/toolkit/lib/api/cloud-assembly/index.ts

@@ -1,5 +1,6 @@
 export * from './cached-source';
 export * from './identity-source';
+export * from './source-builder';
 export * from './stack-assembly';
 export * from './stack-selector';
 export * from './types';