Add /api/v1/admin/domain_allows (#18668)

- `GET /api/v1/admin/domain_allows` lists allowed domains
- `GET /api/v1/admin/domain_allows/:id` shows one by ID
- `DELETE /api/v1/admin/domain_allows/:id` deletes a given domain from the list
  of allowed domains
- `POST /api/v1/admin/domain_allows` to allow a new domain:
  if that domain is already allowed, the existing DomainAllow will be returned

app/serializers/rest/admin/domain_allow_serializer.rb

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