[spring-server] default data fetcher should use Jackson object mapper from spring context

[dariuszkuc]

Library Version
2.0.0-SNAPSHOT

Describe the bug
Currently it is not possible to provide custom Jackson object mapper (e.g. one that includes java date time module) to be used by default function data fetcher (see code below). In order to use custom object mapper users have to create their own instance of data fetcher factory provider.

open class KotlinDataFetcherFactoryProvider {

    private val defaultObjectMapper = jacksonObjectMapper()

    /**
     * Retrieve instance of [DataFetcherFactory] that will be used to resolve target function.
     *
     * @param target target object that performs the data fetching or NULL if target object should be dynamically
     * retrieved during data fetcher execution from [graphql.schema.DataFetchingEnvironment]
     * @param kFunction Kotlin function being invoked
     */
    open fun functionDataFetcherFactory(target: Any?, kFunction: KFunction<*>): DataFetcherFactory<Any> =
            DataFetcherFactories.useDataFetcher(
                FunctionDataFetcher(
                        target = target,
                        fn = kFunction,
                        objectMapper = defaultObjectMapper)
            )
...
}

To Reproduce
Create custom ObjectMapper in spring context (or customize it by using Jackson2ObjectMapperBuilderCustomizer) and notice that it is ignored by KotlinDataFetcherFactoryProvider as it creates its own instance anyway.

Expected behavior
ObjectMapper from spring application context should be used by default data fetcher.