"Sorting signals" functionality in the API spec

[tadasant]

Discussion: #14

Blocked on #87

Although we do not have a hard requirement to implement any sorting signals in the official registry implementation, we do likely want this in the "full" spec that might be used by downstream mirrors of the registry.

This ticket is to define what that shape looks like and incorporate it into the full spec.

[rdimitrov]

Hey @tadasant, the blocker (#87) has since been resolved, but this one itself hasn't gained traction and there's no concrete proposal attached. Going to close as stale; easy to reopen if "sorting signals" turns into a real design with a shape to define.