[Feature Request] Reactive API

### Description

Hello Elastic team,

I wanted to reach out asking for a possible enhancement request please.
This elasticsearch-java project proposes a blocking api. Adding to that it proposes async api. Those two are already great, many thanks.

However, it is currently lacking a reactive api, just to clarify, async does not mean reactive.

Many applications nowadays are reactive (not just async).

Would it be possible to enrich this project with reactive api which respects the reactive manifesto please?