private static void addHttpsRewritingRoutePlanner(HttpClientBuilder builder) {
        if (TransportSecurityUtils.isTransportSecurityEnabled()
                && TransportSecurityUtils.getInsecureMixedMode() != MixedMode.PLAINTEXT_CLIENT_MIXED_SERVER) {
            builder.setRoutePlanner(new HttpToHttpsRoutePlanner());
        }
    }
Creates a client builder with a {@link BasicHttpClientConnectionManager} configured.
This connection manager uses a single connection for all requests. See Javadoc for details.
http-utils/src/main/java/ai/vespa/util/http/hc4/VespaHttpClientBuilder.java