Add extensible custom logic to the domain update flow

This also adds beforeValidation hook to DomainCreateCustomFlow.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139828346

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

@@ -176,6 +176,7 @@ public class DomainCreateFlow implements TransactionalFlow {
         FlagsCreateCommandExtension.class,
         MetadataExtension.class,
         LaunchCreateExtension.class);
+    customLogic.beforeValidation();
     extensionManager.validate();
     validateClientIsLoggedIn(clientId);
     DateTime now = ofy().getTransactionTime();