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/custom/DomainCreateFlowCustomLogic.java

@@ -35,6 +35,12 @@ public class DomainCreateFlowCustomLogic extends BaseFlowCustomLogic {
     super(eppInput, sessionMetadata);
   }
 
+  /** A hook that runs before any validation. This is useful to e.g. add allowable extensions. */
+  @SuppressWarnings("unused")
+  public void beforeValidation() throws EppException {
+    // Do nothing.
+  }
+
   /** A hook that runs at the end of the validation step to perform additional validation. */
   @SuppressWarnings("unused")
   public void afterValidation(AfterValidationParameters parameters) throws EppException {