Add Querydsl integration

[mp911de]

Spring Data repositories that support Querydsl are now supported as DataFetchers returning single objects and iterables including projection support.

interface ArtifactRepositories extends CrudRepository<ArtifactRepository, String>, QuerydslPredicateExecutor<ArtifactRepository> {}

ArtifactRepositories repositories = …;
DataFetcher<ArtifactRepository> single = QuerydslDataFetcherSupport.builder(repositories).single();
DataFetcher<Iterable<ArtifactRepository>> many = QuerydslDataFetcherSupport.builder(repositories).many();

Querydsl predicates are built from the query arguments and can be customized with QuerydslBinderCustomizer to alter comparison (e.g. starts with instead of equals) for individual properties or to generally augment the query. QuerydslDataFetcherSupport also provides projection support (interface projection, DTO projection):

interface BookProjection {

	@Value("#{target.name + ' by ' + target.author}")
	String getName();

	String getAuthor();
}

ArtifactRepositories repositories = …;
DataFetcher<BookProjection> single = QuerydslDataFetcherSupport.builder(repositories).projectAs(BookProjection.class).single();

[rstoyanchev]

I've raised a couple of points around the shape of the public API which might require some moving things around. I'm putting it through for consideration, potentially to make a pass, and I'll have another look and add more comments after when that's settled.

As an extra comment I think it would be good to consider including reactive support as well, perhaps as part of the same builder. I think it's important because we consider how each is exposed from the start and we do support both Spring MVC and WebFlux as foundations.