AWS::Budgets::Budget - NotificationsWithSubscribers-Subscribers

[`AWS::Budgets::Budget`](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-budgets-budget.html) does not handle changing attributes like `NotificationsWithSubscribers-Subscribers` well.

**problem**:
If Subscribers are added/removed from a `AWS::Budgets::Budget` resource updating the stack fails with an error `Resource Budget failed because Error creating budget: my-budget - the budget already exists.`

problem might apply to other attributes too.

**expected behavior** is for the stack not to fail.

**test case**
create a stack with
```
Budget:
Type: AWS::Budgets::Budget
Properties:
Budget:
BudgetName: 'my-budget'
BudgetLimit:
Amount: 100
Unit: USD
TimeUnit: MONTHLY
BudgetType: COST
NotificationsWithSubscribers:
- Notification:
NotificationType: FORECASTED
ComparisonOperator: GREATER_THAN
Threshold: 1
Subscribers:
- SubscriptionType: SNS
Address: !Ref BudgetAlarmTopic
```

update stack by adding/changing the `NotificationsWithSubscribers-Subscribers`

Read the linked AWS::Budgets::Budget documentation, then create the stack from the provided template and update NotificationsWithSubscribers-Subscribers. Confirm the update no longer fails with the “budget already exists” error; investigate whether the same behavior affects other attributes as noted in the issue.

Written by the indexing model from the issue text.