Unified Storage: Don't read before create (#102906)

* Unified Storage: Don't read before create

* test: use the existing test infra

* fix: support pq

We use pgx, but it seems to be wrapped in a pq driver shim, causing the errors to be remapped to pq's type. Weird
situation.

* feat: support CDK backend

* revert: there is a postgres_tests block

* fix(CDK): only check existence on ADDED updates

* fix(CDK): use ReadResource to deal with deleted files

pkg/storage/unified/resource/errors.go

@@ -43,8 +43,8 @@ func AsErrorResult(err error) *ErrorResult {
 		return nil
 	}
 
-	apistatus, ok := err.(apierrors.APIStatus)
-	if ok {
+	var apistatus apierrors.APIStatus
+	if errors.As(err, &apistatus) {
 		s := apistatus.Status()
 		res := &ErrorResult{
 			Message: s.Message,