(optional) directives for generator configuration on types

[jexp]

similar to @model from appsync / graphcool / prisma

[rbramley]

This would be really useful - e.g. some labels in my graph represent reference data, these should not be created/mutated by clients.

Also I may want to annotate schema queries / types to ignore as they are handled by a different back end - something like an @neo4jIgnore directive to filter those out.