Feature Request to configure different regions for different accounts in UpdateStackSet API

### Name of the resource

Other

### Resource name

UpdateStackSet API

### Description

Hello team,

Please consider this as a Feature request to configure different regions for different accounts in the same UpdateStackSet API call.

Current behaviour is that if there are two accounts A and B and regions 1 & 2. If in Account A, stack instances are present in both regions 1 & 2. However for account B there’s only one stack instance in region 1. Now if we want to pass on an identical update, there are two UpdateStackSet calls that need to be made as follows:
```
1st UpdateStackSet: Account A, Account B; region 1
2nd UpdateStackSet: Account A, region 2
```

Note: Providing region2 in the 1st API call would throw a validation error "stack instance not found error", which will not even go to the point of initiating an operation. So there is no scope of making use of fault tolerance option here.

If this feature is released, users can choose to either customly configure their regions for each account given (self managed) or can go with conventional type of configuration like how it is right now.

### Other Details

_No response_