Add a Dropdown Component

Description:
I would like to propose adding a Dropdown component to React Native Paper. Currently, React Native Paper provides several UI components like buttons, cards, and text inputs, but there is no dedicated dropdown component. A dropdown is a widely used component in mobile UIs, ideal for allowing users to select from a list of options in a compact form.

Motivation:
Common UI Need: Dropdowns are a standard UI pattern, and adding this feature will help developers save time by avoiding the need for custom implementations.
Consistency with Other UI Libraries: Many other libraries provide a dropdown component, and having one in React Native Paper will offer a more complete set of UI elements.
Enhanced User Experience: Dropdowns can simplify forms by allowing users to make selections from a list of options without cluttering the screen.

Benefits:
Consistency: A dropdown would align with other components in React Native Paper and enhance the overall library.
Time-Saving: Developers can implement dropdowns quickly without having to build custom solutions.
Customization: Allows developers to create flexible and customizable dropdowns suited to their specific needs.

该 issue 未指定文件或测试；首先检查 React Native Paper 中现有的按钮、卡片和文本输入，以确保一致性。在实现之前，定义下拉菜单的 API、选择行为、平台预期和自定义要求；当这些要求达成一致且组件支持所述用例时，即视为完成。

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