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.

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

java/google/registry/model/domain/fee/FeeUpdateCommandExtension.java

@@ -0,0 +1,21 @@
+// Copyright 2016 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.model.domain.fee;
+
+import javax.xml.bind.annotation.XmlTransient;
+
+/** A fee extension that may be present on domain update commands. */
+@XmlTransient
+public abstract class FeeUpdateCommandExtension extends FeeTransformCommandExtension {}