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/fee/FeeCheckCommandExtensionItem.java

@@ -14,19 +14,22 @@
 
 package google.registry.model.domain.fee;
 
+import javax.xml.bind.annotation.XmlTransient;
+
 /**
  * Interface for individual fee extension items in Check commands. These are derived from the more
  * general query items (which cover Info commands as well), but may also contain a domain name,
  * depending on the version of the fee extension.
  */
-public interface FeeCheckCommandExtensionItem extends FeeQueryCommandExtensionItem {
+@XmlTransient
+public abstract class FeeCheckCommandExtensionItem extends FeeQueryCommandExtensionItem {
 
   /** True if this version of the fee extension supports domain names in Check items. */
-  public boolean isDomainNameSupported();
+  public abstract boolean isDomainNameSupported();
 
   /** The domain name being checked; throws an exception if domain names are not supported. */
-  public String getDomainName() throws UnsupportedOperationException;
+  public abstract String getDomainName();
 
   /** Create a builder for a matching fee check response item. */
-  public FeeCheckResponseExtensionItem.Builder createResponseBuilder();
+  public abstract FeeCheckResponseExtensionItem.Builder<?> createResponseBuilder();
 }