Add admin API for managing canonical e-mail blocks (#19067)

app/serializers/rest/admin/canonical_email_block_serializer.rb

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