Use auto-configured JSON codecs in GraphQlTesterAutoConfiguration

[bclozel]

As seen in spring-projects/spring-graphql#345, our current auto-configuration for the ExecutionGraphQlServiceTester is not aligned with the HttpGraphQlTester auto-configuration behavior. HttpGraphQlTester is based on an existing, auto-configured WebTestClient with JSON codecs. On the other hand, ExecutionGraphQlServiceTester is using its default builder with the default JSON codec configuration.

We should use the newly introduced encoder(Encoder<?>) and decoder(Decoder<?>) methods on the builder with Jackson codecs honoring the application configuration.