Redesign configuration file syntax

[insomniacslk]

The current configuration file syntax is powerful but it exposes too much of the implementation details to the user, and offloads decision chaining in a way that can be hard to predict if the configuration grows significantly. We agree that we want a more user-friendly yet powerful syntax, and that it should be declarative.

@Natolumin and I have thrown a bunch of ideas on https://gist.github.com/Natolumin/2acf759a51becb73080c244b6867d5d9 (these are not formal proposals though) that we would like to discuss more broadly either here or on #coredhcp on gophers.slack.com .

CC @pmazzini @mdlayher for more input, and anyone else feel free to jump in the conversation.