Rework how Query type is defined and customized.

[myronmarston]

Previously, it was defined in GraphQLSDLEnumerator and required an extension to customize it. Defining it in GraphQLSDLEnumerator meant that it was only included in the schema.graphql artifact and was not available to dump in runtime_metadata.yaml. We need to be able to dump resolver runtime metadata for the Query type, and this enables that. Customization can be done via on_built_in_types.

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