fix(toolkit-lib): preserve publishing role for import templates

[sehyun-hwang]

Summary

• copy stack template asset publishing metadata onto generated large template assets
• preserve file publishing role, external ID, additional assume-role options, and destination region when cdk import uploads an override template
• add a regression test for generated large import templates

Root cause

cdk import can generate a large CloudFormation override template and publish it as a synthetic file asset. That asset was created with only bucketName and objectKey, so asset publishing used ambient credentials instead of the bootstrap file publishing role. The fix reuses the stack template asset destination metadata from the synthesized asset manifest.

Validation

• yarn build in packages/@aws-cdk/toolkit-lib reached the package build path through the root build
• root yarn build passed the @aws-cdk/toolkit-lib tests, including the new regression test
• root yarn build later failed in aws-cdk .NET init/migrate tests because dotnet is not installed locally

Issue

n/a

Resolves #1664

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license