RFC: Varlink API for switch-to-configuration-ng

TL;DR: Extend existing switch-to-configuration script with a new flag --varlink-stdio which exposes an Varlink API over stdout/stdin. Audience This is a design discussion for people who work on the activation pipeline, Varlink or any deployment tool interacting with NixOS. Drive-by comments are not helpful there. Motivation switch-to-configuration (abbreviated stc here) is one of the most well-known “public” APIs of the NixOS project, derivatives tend to reproduce similar concepts to enable t...