Support the October 2021 version of the GraphQL spec

A new GraphQL specification has been released in October 2021, see [here](http://spec.graphql.org/October2021/).

I haven't studied it in detail, and I haven't found a press release. But here are some changes I've spotted:

- Interfaces can implement other interfaces
- Non-standard scalars "should" specify a URL to a human-readable specification
- Directives can be repeatable

It's probably a matter of time before other GraphQL services will start making use of this new spec. At that point, graphql-engine will need to support the updated GraphQL spec, at least for the purposes of remote schemas.