Add video support to the Camera API

### What is the problem or limitation you are having?

The camera API lists it can take video currently, but there do not seem to be any methods for it. https://toga.readthedocs.io/en/stable/reference/api/hardware/camera.html

### Describe the solution you'd like

Be able to take and save a video on Andriod and iOS

### Describe alternatives you've considered

N/A

### Additional context

_No response_

Start with the Camera API reference linked in the issue, then inspect the existing Android and iOS camera implementations. Trace how current camera capabilities are exposed and identify the platform-specific entry points for video capture. Done means the API can capture and save video on both Android and iOS, with coverage for the supported behavior.

Written by the indexing model from the issue text.