Schemas should be exposed via API endpoints

[magnusbaeck]

Description

We should expose the GraphQL schema via API endpoints, perhaps /graphql/schema/json and /graphql/schema/sdl. (I have a feeling that there already is such an endpoint since GraphiQL is working but I wasn't able to locate which URL endpoint it uses.)

Motivation

This would allow e.g. end user clients and documentation tools to show relevant information without knowing anything about the service.

Exemplification

Backstage is capable of fetching an API's SDL definition via HTTP and exposing it to the user.

Benefits

Better interaction with third-party tools and clients for end users.

Possible Drawbacks

None.

[magnusbaeck]

See graphql-python/graphene#3 for examples of how to get hold of the different schema representations.