feat!: make json object a scalar

[xsoufiane]

please review 🙏

Motivation

This PR is part of the series that aims to make federation possible in Sangria, and it comes as a response to the federation issue discussed here.

Changes

This PR implements the new contract described in the testkit PR, by ensuring that json objects are GraphQL scalars.

Consequences

• By accepting these changes, any sangria application using the circe marshaller will be able to accept json objects (in variables) as scalars. In particular, let's assume the following schema :

  scalar _Any
  type Query {
    _entities(representations: [_Any!]!): [_Entity]!
  } 
  

  with these changes, we can execute these kind of queries (see how a json object is passed as a scalar):

  {
    "query": "query($representations: [_Any!]!) {...}"
    "variables": {
      "_representations": [
        {
          "__typename": "Product",
          "upc": "B00005N5PF"
        },
        ...
      ]
    }
  }
  

• As for the risks, they are the same as discussed [here]((feat(MarshallingBehaviour)!: allow map to be a scalar sangria-marshalling-testkit#22). Basically, extra care must be taking on the application side to ensure that the scalar json objects are not misused.

Notes for reviewer

At the current moment, tests fail because the PR still requires a new release of the testkit, incorporating the changes proposed here. Once, the new release has been published, another commit should be done updating the build with the new version.

[xsoufiane]

me and @yanns found a way to make federation possible for Sangria without having to change the marshallers and the root sangria lib, and without breaking the existing security level for everyone:) the implementation is done here