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.

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