Add endpoint to print the schema

[andimarek]

It is often very helpful to have an endpoint which returns the whole Schema as printed SDL.

This is in some way similar to the build-in Introspection, but "made for humans" and also the SDL is more rich than Introspection as it can contain comments and directives.

[bclozel]

Do you think this should be bound to the graphql endpoint itself directly?
Or should this be more like of a tooling-related, opt-in endpoint and maybe we should expose that as an actuator endpoint?

[andimarek]

I am not sure about the differences here, but it should be opt in, yes.
The default path should be probably something like /graphql/schema and return text for a GET.

[bclozel]

@andimarek I can't find the proper API to get the schema in a "pretty-printed" fashion. Could you point me to the API we should be using here?

[andimarek]

https://github.com/graphql-java/graphql-java/blob/master/src/main/java/graphql/schema/idl/SchemaPrinter.java is the class to print a GraphQLSchema as SDL