apacheGH-87: [Vector] Add ExtensionWriter (apache#697) (apache#89)

* apacheGH-87: [Vector] Add ExtensionWriter (apache#697)
missed file

Author: xxlaykxx

vector/src/main/codegen/templates/AbstractPromotableFieldWriter.java

@@ -295,7 +295,7 @@ public MapWriter map(boolean keysSorted) {
 
   @Override
   public ExtensionWriter extension(ArrowType arrowType) {
-    return getWriter(MinorType.EXTENSIONTYPE).extension(arrowType);
+    return getWriter(MinorType.LIST).extension(arrowType);
   }
 
   @Override
@@ -325,7 +325,7 @@ public MapWriter map(String name, boolean keysSorted) {
 
   @Override
   public ExtensionWriter extension(String name, ArrowType arrowType) {
-    return getWriter(MinorType.EXTENSIONTYPE).extension(name, arrowType);
+    return getWriter(MinorType.STRUCT).extension(name, arrowType);
   }
 
   <#list vv.types as type><#list type.minor as minor>

vector/src/main/codegen/templates/UnionListWriter.java

@@ -203,8 +203,7 @@ public MapWriter map(String name, boolean keysSorted) {
 
   @Override
   public ExtensionWriter extension(ArrowType arrowType) {
-    writer.extension(arrowType);
-    return writer;
+    return this;
   }
   @Override
   public ExtensionWriter extension(String name, ArrowType arrowType) {