Add return value for ClientSecret

### Name of the resource

AWS::Cognito::UserPoolClient

### Resource name

_No response_

### Description

As part of automation it'd be useful to retrieve the ClientID and ClientSecret both, currently only the Client ID is returned via `Ref`
Given that AWS Keys can be returned (see [return values](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-iam-accesskey.html#aws-resource-iam-accesskey-return-values) there should be an equivalent for this.

The use-case is not for the ALB integration, that works great already.
Thank you,

### Other Details

_No response_

Begin with the AWS::Cognito::UserPoolClient resource and the linked CloudFormation return-values documentation. Confirm how Ref currently exposes the Client ID and determine the intended return path for ClientSecret; done means both values are retrievable without changing the stated ALB integration.

Written by the indexing model from the issue text.