Add mean to denote resource restarts/downtime during plan, similar to ForceNew

Following up on @bflad's suggestion in https://github.com/terraform-providers/terraform-provider-aws/pull/2250#pullrequestreview-75794601, it would be great if Terraform core supported denoting resource restarts/downtime during plan phase, similar to how it denotes recreation (_forces new resource_).

Maybe adding a schema modifier like `ForcesDowntime` like the `ForcesNew` would do it.

For example, adding `ForcesDowntime` to the `instance_type` field of an EC2 instance would warn the user executing the plan that his modification to the configuration, or an out of band modification, will result in downtime of that instance.