AWS::OpenSearchService::Domain - [Enhancement] - Retrieve the Cognito App-Client

### Name of the resource

AWS::OpenSearchService::Domain

### Resource name

_No response_

### Description

Creating an AWS::OpenSearchService::Domain with Cognito as IdentityProvider (to access the Open Search Dashboards) creates an App-Client / UserPoolClient within the given UserPool.

This automatically generated App-Client is not given back, so further Adjustments using it's ID is not easily possible.
To see an use case please look at: https://github.com/aws/aws-cdk/issues/20683
(This is not done by me, I am using CloudFormation, but I do want to do the exact same change to the Identity-Provider, to map Cognito-Authorized-Users to an Open Search User Group with limited rights)

It would be really helpful, to retrieve that App-Client (or at least it's ID) from the stack to be able to adjust the Identity-Provider more easy, than in the linked example.

As a workaround I have currently to fetch the App-Client-Id from bash scripts and pass this to an second stack, just modifying the "AWS::Cognito::IdentityPoolRoleAttachment"

### Other Details

_No response_