Increase usefulness of graphql-prefab-exe by allowing custom resolvers

[jdolle]

graphql-prefab-exe is a powerful concept, but is too rigid at this moment. Ideally, PrefabResolvers can be set up almost like DLLs, so the server package can be modular. The more intuitive, but less convenient version would be offering several executables containing different, commonly used together resolvers so the end user can decide which fits their needs.