webcam capture plugin

It would be great to have a plugin that uses [webrtc camera capabilities](https://github.com/hokein/electron-sample-apps/tree/master/camera) to render a webcam into a plugin. Our [clock plugin](https://github.com/bengreenier/overlayed/blob/master/src/app/plugin/Clock/ClockPlugin.tsx) can likely be used as a base plugin example. If we're able to get things working relatively quickly, it would be good to also consider device enumeration and selection (likely via a config value) so that if a broadcaster has multiple webcams they can be chosen.

Start with src/app/plugin/Clock/ClockPlugin.tsx as the base plugin example and review the linked Electron WebRTC camera sample for capture behavior. Implement a webcam-rendering plugin, then consider device enumeration and config-based selection for broadcasters with multiple webcams.

Written by the indexing model from the issue text.