Validate that a subscription query only contains 1 field

The way that GraphQL-core implements subscriptions means that if you make a subscription query with more than 1 field in it only the resolver for the first field will be called. See: graphql-python/graphql-core@3267c4c/src/graphql/subscription/subscribe.py#L147-L151

There should be some validation on the `subscribe` method on the `Schema` class to make sure that 1 field is only ever passed to a subscription query to prevent confusion.

See https://github.com/graphql-python/graphene/pull/1107#issuecomment-657194024

Start with the Schema class's subscribe method and compare its behavior with graphql-core's subscribe.py at lines 147-151. Determine how a subscription query with multiple fields is currently handled, then add validation so only one field is accepted and verify the resulting behavior with the repository's existing tests.

Rédigé par le modèle d'indexation à partir du texte de l'issue.