AWS::RDS::DBCluster - MasterUsername accessible via Fn::GetAtt

### Name of the resource

AWS::RDS::DBCluster

### Resource name

DBCluster

### Description

Request for an enhancement to the AWS::RDS::DBCluster resource to expose the MasterUsername. This information is already accessible through RDS APIs. Would allow for this type of metadata to be natively exposed within a CFT instead of requiring Customers to write a dedicated CustomResource to expose.

## Scope of Request

New option for an existing attribute is desired that is accessible via AWS RDS APIs. Allows Customers to easily derive the Master Username on a provisioned RDS DB Cluster via CloudFormation.

## Expected Behavior

Fn::GetAtt on a AWS::RDS::DBCluster resource returns the MasterUsername for the resource

### Resources

Resources to assist with investigation:

AWS RDS DescribeDBClusters
https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_DescribeDBClusters.html

DBCluster Object
https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_DBCluster.html

### Other Details

_No response_

Start by reading the AWS::RDS::DBCluster resource behavior and the linked DescribeDBClusters and DBCluster API documentation. Confirm how MasterUsername is exposed by the RDS API and define the Fn::GetAtt result; done means the resource returns that value without requiring a custom resource.

Written by the indexing model from the issue text.