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

定位 SkeletonTable 组件及其相关测试，然后了解当前如何渲染 header 和 body skeleton 行。添加一个布尔选项或行数选项，并将现有的 header 行为设为默认值；验证禁用该选项后 body 行仍保持不变，且相关测试通过。