fix(provider/kafka): replace block() with toFuture().get() in listConsumerGroups

[jeroen92]

Background
When fetching Kafka Consumer Groups from the Jikkou API, we often get 500's returned, e.g.

  "message": "Internal Server Error",
  "errors": [
    {
      "status": 500,
      "error_code": "internal_server_error",
      "message": "Failed to describe consumer groups. Cause IllegalStateException: block()/blockFirst()/blockLast() are blocking, which is not supported in thread default-nioEventLoopGroup-1-43."
    }
  ]
}

Calling Reactor's block() from a Netty event loop thread throws IllegalStateException when Kafka is slow to respond, because Reactor's fast-path (which skips the non-blocking thread check) is only taken when the Mono is already resolved. Under normal conditions the Kafka futures resolve quickly enough to hit that fast-path, making the failure intermittent and correlated with broker latency or availability.

Replace block() with toFuture().get() to bypass Reactor's non-blocking thread detection.

Also explicitly handle ExecutionException and InterruptException. In the former we now unwrap the exception cause so the user-facing error message will be more useful. When catching the InterruptException we re-set the thread's interrupt flag, to prevent having stuck threads during shutdown.