Missing subscriptionAuthorizationFunction for custom subscriptions, alternatively uncooperative pluginHooks with graphql-codegen
During development using postgraphile and graphql-codegen in our organization, we encountered an issue trying to tighten the security around our subscriptions and reduce possible information leaks.

It seems to be impossible to specify a subscriptionAuthorizationFunction using custom subscriptions (not simpleSubscriptions!), and that fact is not clearly communicated in the documentation in our opinion.

After switching to simpleSubscriptions, we encountered another problem, namely that because simpleSubscriptions injects the typedefs for the "listen" field through a pluginHook, which is ignored/not even executed when graphql-codegen autogenerates a schema, graphql-codegen can't find the Subscription and ListenPayload types and throws a really difficult to debug error.

It would be really nice if the subscriptionAuthorizationFunction would be used for custom subscriptions too if it's specified - custom subscriptions play nice with graphql-codegen too.
