Return domain block digests from admin domain blocks API (#29092)

app/serializers/rest/admin/domain_block_serializer.rb

@@ -1,11 +1,15 @@
 # frozen_string_literal: true
 
 class REST::Admin::DomainBlockSerializer < ActiveModel::Serializer
-  attributes :id, :domain, :created_at, :severity,
+  attributes :id, :domain, :digest, :created_at, :severity,
              :reject_media, :reject_reports,
              :private_comment, :public_comment, :obfuscate
 
   def id
     object.id.to_s
   end
+
+  def digest
+    object.domain_digest
+  end
 end