Add Coroutines support

[sdeleuze]

By leveraging https://github.com/konrad-kaminski/spring-kotlin-coroutine or providing a dedicated support.

[sdeleuze]

I think we can add a global Coroutine switch like

application {
    coroutines()
}

When this switch will be enabled, webflux and mongodb modules will provide Coroutines beans like CoroutineWebClient and CoroutineMongoTemplate.

[sdeleuze]

Linked issue to fix as part of this one: ensure interop bewteen Croutines context and Reactor Context.

[sdeleuze]

We have pretty good Coroutines support now, enough for a first milestone, so I close this issue.