refactor: Switch to Two-Phase config generation

Previously the config generation logic directly interacted with in-memory knot data structures. While this works fine, it is very difficult to properly test and also pretty error-prone. As a solution I have introduced some additional data structures which represent a plabsdns specific in memory data structure which is as close as possible to knot config, while still having some sanity checks. This allows for easy unit testing of the first stage of generation, with the second being so simple, it's probably not worth writing tests for