Add aws_elasticache_user_group var

[LuPan2015]

elasticache for redis 6.0 adds ACL function. But now Inputs does not support aws_elasticache_user_group var

[sahiljambhekar]

This should be fairly simple. I can take it up, if users still want this. However, as a consumer of this module, it's fairly easy to add them outside of the module, since it's a separate resource

https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/elasticache_user_group