feat(cognito): add ManagedLoginBranding L2 construct

[ykethan]

Issue # (if applicable)

Closes #.

Reason for this change

Description of changes

This PR adds a new L2 construct for Amazon Cognito ManagedLoginBranding, allowing users to customize the managed login UI experience for their Amazon Cognito User Pools. Added enums for asset categories, color modes, and extensions Added validation for resource properties Added documentation in README.md

References:

AWS CloudFormation resource: AWS::Cognito::ManagedLoginBranding Documentation: Amazon Cognito Managed Login

Describe any new or updated permissions being added

Description of how you validated changes

Added comprehensive unit tests for all scenarios Created an integration test with three different branding configurations:

• Basic settings
• Asset-based branding
• Default Cognito values

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]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 1bd23e5a2a7fd8654d734f81508cdf184ac0886d
• Result: SUCCEEDED
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[alvazjor]

@ykethan Added some comments. But can you also add unit tests for the validations please?

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