feat: added option to use a transactional query invoker

[BlasiusSecundus]

Using the transactional query invoker prevents LazyInitializationException occurring when working with JPA entities across multiple resolvers (thus making individual resolvers Transactional is not sufficient).

By default, this feature is disabled. It can be enabled in application.properties/yaml with:

graphql.query-invoker.transactional=true

[oliemansm]

Can these be included as compileOnly instead so we're not pulling in more dependencies? People that want to use this will probably have these dependencies themselves anyway already, won't they?

[BlasiusSecundus]

Sure, made them compileOnly. transaction-api is also included as testCompile, as it is currently used in one of the tests.

[oliemansm]

@BlasiusSecundus
I've decided to revert this merge because this library already provides a mechanism to add this kind of extension by simply providing your own GraphQLQueryInvoker bean. I want to try to make sure there's enough options in this library that you can override to fit your own needs, but not to include all those separate needs in this library directly.

[vinterdo]

@oliemansm could you provide an example? Also, graphql.query-invoker.transactional=true is described in readme, while not merged/released, causing my confusion why it is not working :)

[oliemansm]

Updated the readme, thanks for pointing it out.

If the default settings are good for you, and you only want to use a customGraphQLQueryInvoker bean, then just add something like this in your app configuration:

@Bean
public GraphQLQueryInvoker queryInvoker(ExecutionStrategyProvider executionStrategyProvider) {
  GraphQLQueryInvoker.Builder builder = GraphQLQueryInvoker.newBuilder()
        .withExecutionStrategyProvider(executionStrategyProvider);
  return new MyCustomQueryInvoker(builder.build());
}

[BlasiusSecundus]

@oliemansm, thanks for the update and the explanation.

I want to try to make sure there's enough options in this library that you can override to fit your own needs, but not to include all those separate needs in this library directly.

That makes a good point. (The PR originated from one such a separate need/solution for that need)