add flag for direct path that reads from system properties (#30588)

* add flag for direct path that reads from system properties

* remove unused imports

* change flag name to something more unique

* remove instance factory and just use boolean

* fix option name format

Author: m-trieu

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/options/DataflowStreamingPipelineOptions.java

@@ -211,6 +211,14 @@ public interface DataflowStreamingPipelineOptions extends PipelineOptions {
 
   void setWindmillServiceStreamMaxBackoffMillis(int value);
 
+  @Description(
+      "If true, Dataflow streaming pipeline will be running in direct path mode."
+          + " VMs must have IPv6 enabled for this to work.")
+  @Default.Boolean(false)
+  boolean getIsWindmillServiceDirectPathEnabled();
+
+  void setIsWindmillServiceDirectPathEnabled(boolean isWindmillServiceDirectPathEnabled);
+
   /**
    * Factory for creating local Windmill address. Reads from system propery 'windmill.hostport' for
    * backwards compatibility.