Supporting the Schema Definition Language (SDL) for list config

[molomby]

The SDL was developed initial by Graphcool but has since been canonised into the GraphQL spec. It provides a way of defining the types and relationships present in a GraphQL schema. This is similar to the role filled by list config in the Keystone ecosystem.

The idea here is to support reading SDL-based lists in on Keystone init, possibly with an "escape hatch" that code gens JS list config. Having a text-based list config format is limiting in some ways (eg. hook syntax? access control needs to be "graphQL-query" based?) but opens the door to dynamical list config changes. Eg, storing list config in the DB and allowing user edit-ability (ie. edit the list config in the Admin UI and reload it) and/or in-app list config versioning.

Other text-based list definition formats (eg. JSON?) could also facilitate this.

[jesstelford]

SDL provides only a subset of the features now available when instantiating a keystone instance.