Remote schema permissions, allow omitting the Query root

Hello,

When defining permissions over a remote schema, we currently must include the Query root.

I can understand that the Query root is needed when directly exposing a GraphQL schema as per the [standard](https://spec.graphql.org/June2018/#sec-Root-Operation-Types) but in the case of Hasura the remote schema can be merged with other APIs or Hasura's own schema so I don't think it makes sense to include the Query root in the scope of permissions.

I think that only the exposed schema should be standard compliant and as a user I find it a bit counter intuitive to have to specify access to a dummy `Query { ok: Boolean! }` when all I want is to grant a role access to one or two remote mutations.

Start by tracing the remote schema permissions flow described in the issue and how it validates the Query root against GraphQL root-operation requirements. The work is done when a role can grant access to remote mutations without defining a dummy Query root, while directly exposed schemas remain compliant with the cited standard; add or update coverage for both cases.

Written by the indexing model from the issue text.