chore(toolkit): export source builder props (#33241)

### Description of changes

Export the prop types from source-builder as intended.

### Describe any new or updated permissions being added

n/a

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

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';