Skeleton Component

**Is your feature request related to a problem? Please describe.**
A skeletal loading component is missing from this library. Previously it was requested and maintainers suggested using react-native-shimmer, however this was 5+ years ago and the suggested library has not been updated since then

**Describe the solution you'd like**
A skeletal loading component in the react-native-paper library

**Describe alternatives you've considered**
React-native-shimmer has been suggested but is no longer maintained.
Other UI libraries such as react-native-elements has this in their library but they are also not being maintained.

**Additional context**
Add any other context or screenshots about the feature request here.

未指定任何文件、测试或入口点。在实现之前，检查 react-native-paper 中现有的组件模式，并明确 skeleton 组件的 API、视觉行为、平台支持和测试预期。

由索引模型根据 Issue 内容生成。