SkeletonTable: add option to hide header skeleton rows

Description

SkeletonTable always renders skeleton rows for the table header. There is no prop to suppress header skeletons while keeping the body skeleton rows.

Use Case

In ansible/ansible-ui, our PageLoadingTable intentionally omits header skeletons because the real table header (with column titles, sort controls, etc.) is already rendered above the loading state by the toolbar/layout. Showing skeleton header rows creates a visual mismatch — users see the real header and a duplicate skeleton header simultaneously.

Proposed Solution

Add a boolean prop (e.g., showHeaderSkeleton defaulting to true, or numberOfHeaderRows defaulting to 1 with 0 meaning none) to control whether header skeleton rows are rendered.

Current Workaround

We maintain a custom PageLoadingTable component that renders PF6 Skeleton components for body rows only, without header skeletons.

Jira Issue: PF-4546

Individua il componente SkeletonTable e tutti i test correlati, quindi leggi come vengono attualmente renderizzate le righe skeleton dell’intestazione e del corpo. Aggiungi un’opzione booleana o un’opzione per il numero di righe, usando come predefinito il comportamento esistente dell’intestazione, e verifica che disabilitarla lasci inalterate le righe del corpo e che i test pertinenti vengano superati.