Validate only one subscription root field

[rmosolgo]

The GraphQL spec says that a subscription operation should only have one selection (after being merged -- the same selection could be spread over different AST nodes) and that it shouldn't be an introspection field.

Here's the spec: https://spec.graphql.org/draft/#sec-Subscription-Operation-Definitions

This isn't implemented in GraphQL-Ruby yet but it could be -- and it would simplify runtime tracking of subscriptions.