Add ability to set DeletionPolicy

[tmclaugh]

Requesting the ability to set a DeletionPolicy policy on DDT autoscaling resources.

As a safety precaution our DynamoDB table resources have a DeletionPolicy of "Retain" so an accidental teardown of production doesn't lose data. In non-production use we spin up and tear down stacks regularly and the end result is we leave tables around without autoscaling enabled and by default many of these tables have high *RC values and end up costing us money until we get around to cleanup. We're also redistributing our code to users who may miss a cleanup step and end up with noticeable unexpected charges.

By being able to retain the autoscaling policy we would limit the cost of these lingering DDT resources.

[sbstjn]

Good point, sounds useful 👍