Add extra logic for all relevant flows

This CL enhances various domain flows (check, create, delete, renew, restore, transfer, update) so that they invoke the appropriate methods on the object implementing the TLD's RegistryExtraFlowLogic (if any). TldSpecificLogicProxy is also updated to invoke RegistryExtraFlowLogic proxy (if any) to fetch the appropriate price. The tests use a made-up extra flow logic object which can be attached to a test TLD to make sure that the proper routines are being invoked.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132486734

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

@@ -14,6 +14,8 @@
 
 package google.registry.model.domain.fee;
 
+import static google.registry.util.CollectionUtils.nullToEmpty;
+
 import google.registry.model.ImmutableObject;
 import java.util.List;
 import javax.xml.bind.annotation.XmlElement;
@@ -51,6 +53,6 @@ public abstract class FeeTransformCommandExtensionImpl
 
   @Override
   public List<Credit> getCredits() {
-    return credits;
+    return nullToEmpty(credits);
   }
 }