Support multiple GraphQL schema files

Created by: eseliger

This PR adjusts all configs/generators/linters that we use to support multiple .graphql files to be stitched together for the final schema. This should help a bit with the MASSIVE schema.graphql file that has grown to around 10k loc by now, by encapsulating different fields of concern into separate files. Using the extend keyword, there is nothing that cannot be expressed with separate files that could have been with a single file, so there are no drawbacks to this method, that I'm aware of.

In a follow-up PR, I will restructure the backend code for this a bit and make it so the enterprise schema is not used in OSS (which gives better introspection, because there is less noise from fields that aren't accessible because OSS) and doesn't require us to implement a defaultXXResolver.