fix(iam): CompositePrincipal synthesizes non-efficient assume role policy

[exoego]

Before this PR, CompositePrincipal synthesize a compact principal for policyFragment method but not for addToAssumeRolePolicy method.
After this PR, CompositePrincipal merges principals whose action is same (e.g. sts:AssumeRole) into a single, compact principal.

Closes #23765

---

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

[gitpod-io]

[aws-cdk-automation]

The pull request linter has failed. See the aws-cdk-automation comment below for failure reasons. If you believe this pull request should receive an exemption, please comment and provide a justification.

A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed add Clarification Request to a comment.

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

PRs must pass status checks before we can provide a meaningful review.

If you would like to request an exemption from the status checks or clarification on feedback, please leave a comment on this PR containing Exemption Request and/or Clarification Request.

[exoego]

Hi maintainers.
I am not sure this change requires new integration tests.
Should I add?

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: f417078
• Result: FAILED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[aws-cdk-automation]

This PR has been in the CHANGES REQUESTED state for 3 weeks, and looks abandoned. To keep this PR from being closed, please continue work on it. If not, it will automatically be closed in a week.

[mrgrain]

@exoego apologies for not getting back to you earlier. This doesn't need an integ test, but the logic would have to be implemented somewhere in aws-iam/lib/private/merge-statements.ts or a related file, not when adding a principal. You are welcome to open a new PR with a different approach.