Add scan region to allow cropping the captured frame

[xulihang]

For frame analyzing like text recognition, we may only need to recognize part of the image.

[xulihang]

I've implemented this with this plugin: https://github.com/tony-xlh/capacitor-plugin-camera-preview

[xulihang]

This plugin also supports scan region: https://github.com/xulihang/capacitor-plugin-camera