@Experimental
    public static CloseableHttpClient classic(final CloseableHttpAsyncClient client, final Timeout operationTimeout) {
        client.start();
        return new ClassicToAsyncAdaptor(client, operationTimeout);
    }
Creates an {@link CloseableHttpClient} facade backed by {@link CloseableHttpAsyncClient}
built internally and acting as a compatibility bridge between the classic APIs based
on the standard {@link java.io.InputStream} / {@link java.io.OutputStream} model
and the underlying asynchronous message transport.
<p>
This feature is considered experimental and may be discontinued in the future.

@param client async client
@param operationTimeout maximum period an operation can block awaiting input / output.

@since 5.5
httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/HttpAsyncClients.java