add draining to output builder, encode draining

Author: stankiewicz

sdks/java/core/src/main/java/org/apache/beam/sdk/values/OutputBuilder.java

@@ -48,5 +48,7 @@ public interface OutputBuilder<T> extends WindowedValue<T> {
 
   OutputBuilder<T> setRecordOffset(@Nullable Long recordOffset);
 
+  OutputBuilder<T> setDraining(@Nullable Boolean drain);
+
   void output();
 }

sdks/java/core/src/main/java/org/apache/beam/sdk/values/WindowedValue.java

@@ -52,6 +52,9 @@ public interface WindowedValue<T> {
   @Nullable
   Long getRecordOffset();
 
+  @Nullable
+  Boolean isDraining();
+
   /**
    * A representation of each of the actual values represented by this compressed {@link
    * WindowedValue}, one per window.