Replace AkkaHttpBackend with AsyncHttpClientFutureBackend

[gskrobisz]

In 5205163 we added readTimeout to HttpFlinkClient.findJobsByName.
Akka does not support such setting and sttp uses idleTimeout as readTimeout, see sttp.client3.akkahttp.AkkaHttpBackend :

  private def connectionSettings(r: Request[_, _]): ConnectionPoolSettings = {
    //...
    connectionPoolSettingsWithProxy
    .withUpdatedConnectionSettings(_.withIdleTimeout(r.options.readTimeout))
  } 

And that idle/readTimeout means that after timeout connection is killed via RST (see also akka/akka-http#3346).
Therefore, to avoid broken sessions of findJobsByName, when HttpFlinkClient is created we should replace use AsyncHttpClientFutureBackend instead of AkkaHttpBackend.

[arkadius]

Please remember to change scenarioStateIdleTimeout parameter name and update docs related with this