Add extensible custom logic to the domain check flow

This also fixes up a hook on the domain create flow custom logic to use a single
parameter, which is the general pattern we want to use going forward. It also
establishes a pattern for custom logic being able to add extensions.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139362230

java/google/registry/flows/domain/DomainCreateFlow.java

@@ -309,9 +309,10 @@ public class DomainCreateFlow implements TransactionalFlow {
             DomainCreateFlowCustomLogic.BeforeSaveParameters.newBuilder()
                 .setNewDomain(newDomain)
                 .setHistoryEntry(historyEntry)
+                .setEntityChanges(
+                    EntityChanges.newBuilder().setSaves(entitiesToSave.build()).build())
                 .setYears(years)
-                .build(),
-            EntityChanges.newBuilder().setSaves(entitiesToSave.build()).build());
+                .build());
     persistEntityChanges(entityChanges);
 
     return responseBuilder