corrections for types / pruning unnecessary types

[grokspawn]

We took another look at the existing fields in the API and came up with a set of changes which would enhance clarity and reduce complexity of the types.

• Instead of using unionDiscriminator directives + validations for discriminated unions, use CEL expressions, similar to: https://github.com/operator-framework/operator-controller/blob/75bb03dda3a49a2b6813243fee7152886f8919b8/api/v1alpha1/clusterextension_types.go#L80
• delete config/base/crd/patches/catalog_validation.yaml
• delete ClusterCatalogStatus.ObservedGeneration and uses. In a future effort we will want to evaluate new-available-content by comparing new/last hashes, so this is the wrong direction.
• enum validation statement for CatalogSource.SourceType should use the field name Image instead of the JSON name
• remove omitempty from ClusterCatalogSpec.Priority so that printcols will display '0' when defaulting
• use ResolvedImageSource.Ref instead of ResolvedRef, and delete ResolvedRef
• delete ResolvedImageSource.LastUnpacked as this duplicates tracking in the root (and redirect any references to use the root)
• rename status.source.image.lastPollAttempt as lastSuccessfulPollAttempt to clarify intent

[grokspawn]

started a draft PR for this work, but gotten pulled off to work on other things, so reverting status and unassigning. Sorry!

[LalatenduMohanty]

Closing the issue as #384 has been merged.