Launch in a "portal" mode

### Problem

Sometimes apps want to be able to ask the user to take a photo without needing direct access to the camera themselves or wanting to implement their own full camera interface.

### Proposal

A "take a photo" portal mode for camera where apps can call the camera over dbus, it will launch in a dialog/portal mode with a "Done" button etc, and then pass back the photo in the same way the filechooser portal would

### Prior Art (Optional)

_No response_