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/RegistryExtraFlowLogic.java

@@ -27,6 +27,7 @@ import org.joda.time.DateTime;
  * Interface for classes which provide extra registry logic for things like TLD-specific rules and
  * discounts.
  */
+@Deprecated
 public interface RegistryExtraFlowLogic {
 
   /** Gets the flags to be returned for application info commands. */