Retrieve only the value from AWS::CloudFormation::WaitCondition in the template.

### Name of the resource

AWS::CloudFormation::WaitCondition

### Resource name

_No response_

### Description

As shown below, Currently we can only reference the whole JSON data object from a Wait Condition.
```
"WaitConditionData" : {
"Value" : { "Fn::GetAtt" : [ "mywaitcondition", "Data" ]},
"Description" : "The data passed back as part of signalling the WaitCondition"
},
```
Below is a sample JSON object which can be received.
```
{"S3Key":"acbd/pqr.zip"}
```
But there is no way to only reference the value "acbd/pqr.zip" without the key.

It will be helpful for users if we can also "GetAtt" the value in the key value pair.

Something like as follows:
```
"WaitConditionData" : {
"Value" : { "Fn::GetAtt" : [ "mywaitcondition", "Data.S3Key" ]},
"Description" : "The data passed back as part of signalling the WaitCondition"
},
```

### Other Details

_No response_

Review the AWS::CloudFormation::WaitCondition example and proposed Fn::GetAtt Data.S3Key syntax in this issue. Determine the supported way to retrieve a value from the returned JSON while preserving existing whole-object access; done means the behavior is defined and documented or validated.

Written by the indexing model from the issue text.