Support returning `Flux` of entities from `@EntityMapping` methods

[rstoyanchev]

@EntityMapping methods can load entities of a given type in batch mode by returning List<T>, Mono<List<T>>, CompletableFuture<List<T>>, and others. We can also support Flux<T>, and adapt via Flux#collectList.

Based on #922 (comment) by @gianielsevier.