Auto-configure DataLoaderRegistrar components

This commit configures all `DataLoaderRegistrar` application components
with the `GraphQlService`. This allows applications to contribute
`DataLoader` instances without using the annotated-controller
programming model.

Closes gh-189

spring-graphql-docs/src/docs/asciidoc/boot-starter.adoc

@@ -258,6 +258,10 @@ public class BookController {
 }
 ----
 
+`BatchLoaderRegistry` is itself a `DataLoaderRegistrar`, as it will register `DataLoader` instances for each request.
+Applications can also contribute other `DataLoaderRegistrar` instances as components - they will be configured with
+the `ExecutionGraphQlService` automatically.
+
 
 
 
@@ -451,6 +455,7 @@ By default, `@GraphQlTest` limits scanning to the following beans:
 
 - `@Controller`
 - `RuntimeWiringConfigurer`
+- `DataLoaderRegistrar`
 - `JsonComponent`
 - `Converter`
 - `GenericConverter`