Support CameraClipPreview for Nest Doorbell (battery)
`getEventImage()` in Camera.ts only queries `CameraEventImage`. However, on the Nest Doorbell (battery), `CameraClipPreview` is used instead. https://developers.google.com/nest/device-access/traits/device/camera-clip-preview describes the preview as

> A clip preview is a 10 frame video file in mp4 format. You can choose to show the mp4 video file, or transcode it to another format (for example, an animated gif) in order to show a complete 10 frame sequence. The device client capabilities should be considered when selecting a clip preview method.

Since it doesn't seem like Homebridge supports animated previews, it would be nice to pick one of the frame, convert it to JPEG, and attach it to `this.image` so `getSnapshot()` can use it. Maybe the frame number can be a user configurable option.