Addition of PlatformDropdownButton and PlatformDropdownMenu

[TeeWrath]

Description

During the development of a project, I identified the absence of dropdown button functionality. To address this, I propose the implementation of a DropdownButton widget and a DropdownMenu to enhance the user interface and functionality.

Implementation Plan

To achieve a cross-platform solution, I will utilize the DropdownButton widget from the Material library for Android-compatible interfaces and the CupertinoPicker from the Cupertino library for iOS-compatible interfaces. These widgets will be integrated to create a unified, platform-adaptive dropdown component.

Subsequently, I will extend the implementation to include support for a DropdownMenu, ensuring comprehensive functionality across platforms.

would love to hear your thoughts @aqwert