Add extra flow logic hooks for info and update

This CL adds the hooks necessary to implement TLD-specific flow info and update flow logic. Usage of the hooks follows in a separate CL.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130108832

java/google/registry/model/domain/flags/FlagsInfoResponseExtension.java

@@ -30,4 +30,10 @@ import javax.xml.bind.annotation.XmlRootElement;
 public class FlagsInfoResponseExtension implements ResponseExtension {
   @XmlElement(name = "flag")
   List<String> flags;
+
+  public static FlagsInfoResponseExtension create(List<String> flags) {
+    FlagsInfoResponseExtension extension = new FlagsInfoResponseExtension();
+    extension.flags = flags;
+    return extension;
+  }
 }