fix(appconfig): sourcedConfiguration doesn't use retrievalRole (#30733)

### Issue # (if applicable)

Closes #30609

### Reason for this change

To refactor the retrievalRole creation logic.



### Description of changes



### Description of how you validated changes

Unit tests:
1. configuration with retrievalRole undefined from bucket source should create a new role
2. configuration with retrievalRole defined should NOT create a new role and should use the passed role for the retrievalRoleArn
3. configuration with retrievalRole undefined from CodePipeline source should NOT create a new role

Integ test:
1. update the existing integ test to assert a use case where the retrievalRole is provided.

### 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*

Author: pahud