AWS::RDS::DBProxy - DbiResourceId accessible via Fn::GetAtt

### Name of the resource

AWS::RDS::DBProxy

### Resource name

_No response_

### Description

This is related to https://github.com/aws-cloudformation/cloudformation-coverage-roadmap/issues/105
`AWS::RDS::DBProxy` resources have a `ResourceId` similar to `AWS::RDS::DBInstance`, which is used inside `arns`.
The motivation is exactly the same as in https://github.com/aws-cloudformation/cloudformation-coverage-roadmap/issues/105

Specifically:

```yaml
"DbProxyInstance": {
"Type": "AWS::RDS::DBProxy",
"Properties": { }
},
"DBUserPolicy": {
"Type": "AWS::IAM::ManagedPolicy",
"Properties": {
"Description": "blah blah not relevant",
"PolicyDocument": {
"Version": "2012-10-17",
"Statement": [
{
"Action": [ "rds-db:connect" ],
"Effect": "Allow",
"Resource": [
{ "Fn::Sub": "arn:aws:rds-db:${AWS::Region}:${AWS::AccountId}:dbuser:prx-n0q2srwsyoc69jd0uk/db_user_name" }
]
}
]
}
}
}
}
```

I would like to change the Resource entry like this:

```
"Fn::Sub": [
"arn:aws:rds-db:${AWS::Region}:${AWS::AccountId}:dbuser:${DbiResourceId}/db_user_name",
{ "DbiResourceId": { "Fn::GetAtt": [ "DbProxyInstance", "DbiResourceId" ] } } ]
```

### Other Details

_No response_

Start by reviewing the related coverage-roadmap issue 105 and the AWS::RDS::DBProxy resource described here. Confirm how DbiResourceId should be exposed through Fn::GetAtt and usable in the shown Fn::Sub policy example; done means the requested resource attribute is supported and documented for this resource.

Written by the indexing model from the issue text.