Add support for kotlin coroutine suspend functions to WebFluxLinkBuilder

Currently the only solution that allows HATEOAS to interoperate with suspend coroutine functions is to wrap all output into Mono and Flux (Flow seems to work for some reason too, but it's probably a bad idea to use it without understanding why it works).

In short, implement the following prototype:

fun <reified T> WebFluxLinkBuilder.linkTo(block: suspend T.() -> Any?)

I realize now there's a second problem identified here, which is that the boilerplate for methodOn can be eliminated with reified generics in Kotlin. Perhaps another task can be to add Kotlin extensions when possible?

Start at WebFluxLinkBuilder and inspect its existing linkTo entry points alongside the current Mono and Flux handling. Implement and validate the proposed suspend-function linkTo prototype so coroutine functions no longer need their output wrapped in Mono or Flux.