[BEAM-11934] Add enable_file_dynamic_sharding to allow DataflowRunner…

… users to opt-in to dynamic sharding logic (apache#17095)

* [BEAM-11934] Remove DataflowRunner override for runner-determined sharding

Since apache#14450, this override artificially constrains dataflow's ability to use autosharding.

* Revert "[BEAM-11934] Remove DataflowRunner override for runner-determined sharding"

This reverts commit f575256.

* change to be an experiment

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

@@ -523,10 +523,12 @@ private List<PTransformOverride> getOverrides(boolean streaming) {
 
       overridesBuilder.add(KafkaIO.Read.KAFKA_READ_OVERRIDE);
 
-      overridesBuilder.add(
-          PTransformOverride.of(
-              PTransformMatchers.writeWithRunnerDeterminedSharding(),
-              new StreamingShardedWriteFactory(options)));
+      if (!hasExperiment(options, "enable_file_dynamic_sharding")) {
+        overridesBuilder.add(
+            PTransformOverride.of(
+                PTransformMatchers.writeWithRunnerDeterminedSharding(),
+                new StreamingShardedWriteFactory(options)));
+      }
 
       overridesBuilder.add(
           PTransformOverride.of(