Split XJC code into separate eppbindings subproject (#3244)

The generated XML / XJC code pretty much never changes but because
everything is lumped together in one compile task, we had to recompile
this every time anything else in core/ changes. It's technically
possible to use a separate source set for the XJC code while having it
remain in core/, but splitting it into a separate subproject gives
better separation and makes the caching / dependency graph clearer.

Pretty much all, except for the caching configuration, of the new
xjc/build.gradle is taken from the previous core/build.gradle.

eppbindings/src/main/java/google/registry/xml/PeriodAdapter.java

@@ -0,0 +1,37 @@
+// Copyright 2017 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.xml;
+
+import static com.google.common.base.Strings.isNullOrEmpty;
+
+import jakarta.xml.bind.annotation.adapters.XmlAdapter;
+import java.time.Period;
+import javax.annotation.Nullable;
+
+/** Adapter to use Joda {@link Period} when marshalling XML. */
+public class PeriodAdapter extends XmlAdapter<String, Period> {
+
+  @Nullable
+  @Override
+  public Period unmarshal(@Nullable String periodString) {
+    return isNullOrEmpty(periodString) ? null : Period.parse(periodString);
+  }
+
+  @Nullable
+  @Override
+  public String marshal(@Nullable Period period) {
+    return period == null ? null : period.toString();
+  }
+}