fix(rds): Failed to create a Database Instance with Kerberos authentication configured (#28601)

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.

```diff
-        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.

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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: badmintoncryer