Add FlowMetadata (containing isSuperuser) to custom flow logic

This also bypasses signed mark validation during domain creation if
the flow is being executed as superuser.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=145435268

java/google/registry/flows/FlowModule.java

@@ -259,6 +259,11 @@ public class FlowModule {
         .setResultFromCode(Result.Code.SUCCESS);  // Default to success.
   }
 
+  @Provides
+  static FlowMetadata provideFlowMetadata(@Superuser boolean isSuperuser) {
+    return FlowMetadata.newBuilder().setSuperuser(isSuperuser).build();
+  }
+
   /** Wrapper class to carry an {@link EppException} to the calling code. */
   static class EppExceptionInProviderException extends RuntimeException {
     EppExceptionInProviderException(EppException exception) {