Close Nessie client

[findepi]

This doesn't fix anything yet, because NessieIcebergClient.close calls HttpApiV1.close, which calls RestV1Client.close, which calls, JavaHttpClient.close, which nulls out JavaHttpClient.client, without closing it.

Found by #21913

[findepi]

@ajantha-bhat can you please fix JavaHttpClient.close?

[ajantha-bhat]

@findepi: Thanks for pointing out. I will add the below code before nullifying the client at Nessie side.

    // Close associated resources
    client.executor().ifPresent(executor -> {
      if (executor instanceof ExecutorService) {
        ((ExecutorService) executor).shutdown();
      }
    });