In order to support an IPv6 desktop native VPN on macOS, we need to be able to configure DNS settings to direct domain queries with the suffix .coder to the DNS server embedded in tailnet (#14718). We want to do this using the native NetworkExtension APIs. Tailscale's wgengine uses a dns.OSConfigurator to set DNS configuration. We need to build a new implementation of this interface that uses the CoderVPN protocol (#14731) to send NetworkSettings to the manager, which then calls the native APIs.