This commit adds the auto-configuration for setting up the base Spring
GraphQL infrastructure. Because GraphQL doesn't depend on any particular
transport, we must have a separate configuration for creating:
* the `GraphQlSource`, which holds the schema and the `GraphQL` instance
* the `GraphQlService` for executing incoming requests
* the `BatchLoaderRegistry` for batch loading support
* the `AnnotatedControllerConfigurer` for supporting the annotated
controllers programming model
This comes with a starting point for the `"spring.graphql.*"`
configuration properties; we can now configure the locations and file
extensions of GraphQL schema files we should load and configure at
startup.
See gh-29140
de808834f5