fix(rds): Failed to create a Database Instance with Kerberos authentication configured

[badmintoncryer]

This PR resolves an issue where deploying an RDS instance configured with Kerberos authentication fails.
When domainRole is undefined, CDK creates a default IAM role. However, this role lacks the necessary principals, leading to deployment failure. To resolve this, the necessary principals have been added to the role.

-        assumedBy: new iam.ServicePrincipal('rds.amazonaws.com'),
+        assumedBy: new iam.CompositePrincipal(
+          new iam.ServicePrincipal('rds.amazonaws.com'),
+          new iam.ServicePrincipal('directoryservice.rds.amazonaws.com'),
+        ),

Closes #28600.

---

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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[go-to-k]

Looks good.

[badmintoncryer]

@go-to-k Thanks!

[GavinZZ]

Changes look good. Thanks for addressing this issue!

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 9fe162e
• Result: SUCCEEDED
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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).