Custom IAM role/policy for build stage for source code

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**Tell us about your request**
Currently App Runner does not work with retrieving code from private CodeArtifact repositories during the build process. To access a pip repository on CodeArtifact, one must login via aws cli, which requires the right IAM credentials. However, App Runner uses a custom created role in the build process (which cannot access CodeArtifact) and not the provided IAM role.
To fix this issue, it should be possible to supply an IAM role/policy for the build process as well.

**Describe alternatives you've considered**
As an alternative, one could supply access keys for an IAM user with access to CodeArtifact, however this approach has security concerns and cannot be implemented via cloudformation.