Create a utility to simplify aggregating reactive links

[gregturn]

This pattern kind of gets old:

return Mono.zip(selfLink, aggregateLink, addLink, deleteLink) //
		.map(links -> Links.of(links.getT1(), links.getT2(), links.getT3(), links.getT4()))
		.flatMap(links -> this.repository.findById(id) //
				.map(item -> new EntityModel<>(item, links)));

The first two lines are Reactor boilerplate to take several WebMvcLinkBuild.linkTo....toMono() calls and chain them together into a Mono<Links> object.

If we had a utility method(s), then this could be vastly simplified.