Simplify the fee extensions.

I added shared base classes to all of the Fee extension types that
make it possible to fully ignore the version in the flows. (You
ask for a FeeCreateCommandExtension, for example, and you get one
without having to worry about which). This is an improvement over
the old code that asked you to provide a list of possible fee
extensions and then ask for the first one in preference order.

As part of this I was able to make the Fee implementation a bit
simpler as well.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=137992390

java/google/registry/model/domain/fee06/FeeCheckCommandExtensionItemV06.java

@@ -16,22 +16,47 @@ package google.registry.model.domain.fee06;
 
 import com.google.common.base.Optional;
 import google.registry.model.domain.fee.FeeCheckCommandExtensionItem;
-import google.registry.model.domain.fee.FeeCheckResponseExtensionItem;
-import google.registry.model.domain.fee.FeeQueryCommandExtensionItemImpl;
+import google.registry.model.domain.fee.FeeExtensionCommandDescriptor;
 import javax.xml.bind.annotation.XmlType;
 import org.joda.money.CurrencyUnit;
 import org.joda.time.DateTime;
 
 /** An individual price check item in version 0.6 of the fee extension on Check commands. */
 @XmlType(propOrder = {"name", "currency", "command", "period"})
-public class FeeCheckCommandExtensionItemV06
-    extends FeeQueryCommandExtensionItemImpl implements FeeCheckCommandExtensionItem {
+public class FeeCheckCommandExtensionItemV06 extends FeeCheckCommandExtensionItem {
 
   /** The fully qualified domain name being checked. */
   String name;
 
   CurrencyUnit currency;
 
+  /** The command being checked.  */
+  FeeExtensionCommandDescriptor command;
+
+  /** The name of the command being checked. */
+  @Override
+  public CommandName getCommandName() {
+    return command.getCommand();
+  }
+
+  /** The command name before being parsed into an enum, for use in error strings. */
+  @Override
+  public String getUnparsedCommandName() {
+    return command.getUnparsedCommandName();
+  }
+
+  /** The phase of the command being checked. */
+  @Override
+  public String getPhase() {
+    return command.getPhase();
+  }
+
+  /** The subphase of the command being checked. */
+  @Override
+  public String getSubphase() {
+    return command.getSubphase();
+  }
+
   @Override
   public boolean isDomainNameSupported() {
     return true;
@@ -48,7 +73,7 @@ public class FeeCheckCommandExtensionItemV06
   }
 
   @Override
-  public FeeCheckResponseExtensionItem.Builder createResponseBuilder() {
+  public FeeCheckResponseExtensionItemV06.Builder createResponseBuilder() {
     return new FeeCheckResponseExtensionItemV06.Builder();
   }