Add support for devices in devcontainer-feature.json

While implementing a [Dev Container feature for Tailscale](https://github.com/tailscale/codespace/pull/6), I discovered that there's no way to have the feature add the equivalent of `runArgs: ["--device=/dev/net/tun"]`, even though you can set `privileged` (which would work, but gives way more access than needed). Would it be possible to add this to the feature metadata spec?