### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

When attempting to list roles where permission boundaries are attached the response cannot be serialized 

`aws --endpoint-url=http://localhost:4566 iam list-roles --region ap-southeast-2`
```
An error occurred (InternalError) when calling the ListRoles operation (reached max retries: 2): exception while calling iam.ListRoles: Invalid type when serializing AttachedPermissionsBoundary: '<Element 'member' at 0xffff8275ef20>' cannot be parsed to structure.
```

### Expected Behavior

Expect that all roles are listed

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run localstack/localstack

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

* Create an IAM policy
* Create an IAM role which used the policy as a permission boundary
* Attempt to list all roles with `aws --endpoint-url=http://localhost:4566 iam list-roles --region ap-southeast-2`


### Environment

```markdown
- OS: MacOS
- LocalStack: latest
```


### Anything else?

_No response_
self, context: RequestContext, user_name: existingUserNameType = None
    ) -> GetUserResponse:
        response = call_moto(context=context)
        moto_user_name = response["User"]["UserName"]
        moto_user = get_iam_backend(context).users.get(moto_user_name)
        
        if not moto_user and not user_name:
            access_key_id = extract_access_key_id_from_auth_header(context.request.headers)
            sts_client = aws_stack.connect_to_service(
                "sts",
                aws_access_key_id=access_key_id,
                aws_secret_access_key=INTERNAL_AWS_SECRET_ACCESS_KEY,
            )
            caller_identity = sts_client.get_caller_identity()
            caller_arn = caller_identity["Arn"]
            if caller_arn.endswith(":root"):
                return GetUserResponse(
                    User=User(
                        UserId=context.account_id,