[V4] Provide a method to download the schema in GraphQL or JSON format

Introspection provides the ability for other tools to ask questions to the GraphQL API. However, sometimes you have a tool that requires a GraphQL schema or JSON schema file (like Postman) which doesn't have its own introspection tool built in and you don't want to set up a toolchain just to get this data.

The proposal is to add a method on the GraphQL configuration page, similarly to the validation tab, that allows a developer to download the current schema either as `.graphql` file (which merges and simplifies all enabled `.graphqls` files) or as `.json` file.