fix(appconfig): sourcedConfiguration doesn't use retrievalRole

[pahud]

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

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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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

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[Leo10Gama]

Just one minor nit about one of the comments, but otherwise LGTM!

[Leo10Gama]

Thank you! LGTM

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