Creates a policy assignment by ID.

Policy assignments are inherited by child resources. For example, when you apply a policy to a resource group that policy is assigned to all resources in the group. When providing a scope for the assignment, use '/subscriptions/{subscription-id}/' for subscriptions, '/subscriptions/{subscription-id}/resourceGroups/{resource-group-name}' for resource groups, and '/subscriptions/{subscription-id}/resourceGroups/{resource-group-name}/providers/{resource-provider-namespace}/{resource-type}/{resource-name}' for resources.