Allow root query type to be optional to support federation implementations better #667

I want to suggest an idea and checked that ...

• ... to my best knowledge, my idea wouldn't break something for other users
• ... the documentation does not mention anything about my idea
• ... there are no open or closed issues that are related to my idea

Description

Hi there, we are using graphql-java-kickstart to implement GQL microservices in conjunction with apollo federation. This basically works very good.

However, I'd like to propose the enhancement to make the root query type also optional (by configuration).

Background:
(at least in our setup) some microservices only extend types defined by other services and add fields to existing types. They never are requested "directly". Basically there is no need to define the query type at all for those services. Currently graphql-java-tools throws an exception when starting up like this (graphql.kickstart.tools.SchemaClassScannerError: Type definition for root query type 'Query' not found!).

Use Cases

Implementing services of a federation can easily add separate fields to entites without the need to define a top level query.