Cross-resource constraints

We're building an S3 bucket notification resource, to solve a [long-standing problem](https://read.acloud.guru/cloudformation-is-an-infrastructure-graph-management-service-and-needs-to-act-more-like-it-fa234e567c82). Each item in the notification configuration for a bucket is a separate resource. However, the rules for bucket notifications says they can't overlap in prefix+events. So two resources can share a prefix if they subscribe to different events, or if they share events they can't share a prefix. If someone creates set of resources that violates these constraints, the S3 API call will return an error at some point in the process, which we'll surface to the user. But it got me thinking if these kinds of constraints could be expressed in the schema. It could be something like `"uniquePropertySets": {"SomeNameForThis": [ '#/properties/SomeScalarProperty', '#/properties/SomeSetValuedProperty']}`

The issue names no files, tests, or entry points; begin by reviewing the resource-schema design and how schemas currently express validation constraints. Done would be an agreed, documented schema representation for cross-resource uniqueness constraints, with implementation scope and validation behavior defined.

Written by the indexing model from the issue text.