feat: support external IDs when assuming a role with a custom resource

[crowecawcaw]

Reason for this change

We want to use the AwsCustomConstruct for making calls in another account while assuming a role. As a security mitigation, our cross account role requires an external ID. This change allows the AwsCustomConstruct to optionally use external IDs.

Description of changes

Add assumedRoleExternalId as an optional AwsCustomConstruct parameter and use it when making cross-account calls.

Describe any new or updated permissions being added

None

Description of how you validated changes

Unit and integ tests.

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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[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Features must contain a change to a README file.
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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 57a4186
• Result: FAILED
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