AWS::ElastiCache::User - [Enhancement] - Ability to add a user to a group from a different stack

### Name of the resource

AWS::ElastiCache::User

### Resource name

_No response_

### Description

I have an application where multiple instances of the application (e.g. dev and test) share the same ElastiCache cache. So, I have one stack based on one template for the cache, and multiple stacks based on another template for each instance. I would like to define the user group in the cache template and a user and its group membership in the application instance template. A new resource type (AWS::ElastiCache::UserGroupMembership) that adds a user to a group would be best, but a UserGroups property on the AWS::ElastiCache::User resource would also suit my needs.

### Other Details

This will also require that the UserIds property of the AWS::ElastiCache::User resource not show as drifted if other users have been added to the group.

Review the AWS::ElastiCache::User resource and the proposed AWS::ElastiCache::UserGroupMembership resource or UserGroups property. Determine how cross-stack group membership should work and how UserIds can avoid false drift when other users are added. Done means a user can be assigned to a group from another stack without incorrect drift reporting.

Written by the indexing model from the issue text.