Configure FormattingConversionService in GraphQlAutoConfiguration

[bclozel]

As of spring-projects/spring-graphql#271, the AnnotatedControllerConfigurer will not allow to set the ConversionService directly, but rather let developers register formatters to it.

We should update the GraphQL auto-configuration to expose this feature . Unlike Web MVC, there cannot be a web-specific ConversionService instance and given the transport-agnostic nature of GraphQL we should not apply the web-specific converters there. Instead we could apply the ApplicationConversionService formatters and/or open it for custom registration of formatters through a callback.