Add admin API for managing e-mail domain blocks (#19066)

app/serializers/rest/admin/email_domain_block_serializer.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class REST::Admin::EmailDomainBlockSerializer < ActiveModel::Serializer
+  attributes :id, :domain, :created_at, :history
+
+  def id
+    object.id.to_s
+  end
+end