feat: Allow UI Customization

**Description:**
I am requesting an enhancement for the @capacitor/barcode-scanner plugin to support UI customization, allowing developers to overlay web content on the camera view while scanning.

**Current Behavior:**
The current implementation of the @capacitor/barcode-scanner plugin launches a new activity (on Android) or a new view controller (on iOS) dedicated to barcode scanning. This approach restricts the ability to customize the WebView's user interface during the scanning process, as the user is taken out of the main web-based context into a separate native interface.

**Requested Feature:**
I propose adding support to the plugin to allow the camera view to be rendered behind the WebView. This would enable developers to customize the UI by overlaying HTML/CSS elements on top of the camera feed, providing a seamless and integrated user experience within the existing web-based UI.

**Sample:**