projectreactor.io support

### Description
Please make support the reactive approach in the client.
The main reason for this request is that it is hard to integrate this client into the Spring WebFlux framework.
There is a workaround for wrapping `CompletableFuture` into the `Mono` object but in the reactive environment, it will produce poor performance.

Start by reviewing the Java client's CompletableFuture-based API and the integration requirements of Spring WebFlux and Project Reactor. Define which client operations need native reactive support and what successful use in WebFlux should look like; the issue does not name files or tests.

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