chore(spec2cdk): ensure strings are passed to attributes

[leonmk-aws]

Issue # (if applicable)

Closes #.

Reason for this change

In some languages for which jsii cannot properly generate unions like Go, if a user passes the wrong property (i.e an ILayerRef instead of an IRoleRef), the error message will be unclear:

panic: ValidationError: Resolution error: Resolution error: Trying to resolve() a Construct at /Resources/${Token[GoAppStack.SampleFunction.LogicalID.139]}/Properties/role/node..

Description of changes

Add a check to ensure that all unions have been resolved to a string and throw an error if not, at runtime it will look like:

panic: TypeError: Property role should be one of iam.IRoleRef | string

Description of how you validated changes

Ran locally, updated snapshot tests. Unit test will be added once the relationships are enabled for lambda.

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

[aws-cdk-automation]

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