feat: Add support for the DropdownMenu widget

**Description**

Add support for the [DropdownMenu widget](https://api.flutter.dev/flutter/material/DropdownMenu-class.html).

**Requirements**

- [ ] Create widget parser
- [ ] Add an example in the [Stac Gallery App](https://github.com/StacDev/stac/tree/dev/examples/stac_gallery)
- [ ] Add documentation in the [Stac Docs website](https://github.com/StacDev/stac/tree/dev/website/docs)