UpdatePolicy "Always"

Currently, CloudFormation only triggers resources to perform updates when their parameters have changed. However, some custom resource use cases (like referencing external resources and providing values as return values) lead to the desire to _always_ update a resource. Today, I accomplish this with tooling, inserting a timestamp as a parameter and updating it whenever the stack is updating, triggering that custom resource. But there's already a notion of `UpdatePolicy` in CloudFormation. There should be the ability to say something like `{"Always": true}` in your `UpdatePolicy` and have CloudFormation always trigger the resource update.

Separately, it would be good for custom resources to set the default UpdatePolicy during resource creation, so that the custom resource itself can say "please call me again whenever the template is updated".