Namespace storage purchase api

What does this MR do?

Following along with !34746 (merged) this MR is part of #223225 (closed).

You can see in this proposal how and when the API will be used: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/staging/doc/proposal-storage-limit.md#scenario-5-paid-plan-with-or-without-extra-storage

NOTE: I've not added docs for the API since it will be used by customer portal only. We also don't expose the API for CI minutes yet in our docs.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

ee/app/models/ee/namespace.rb

@@ -26,10 +26,12 @@ module Namespace
       attr_writer :root_ancestor
 
       has_one :namespace_statistics
+      has_one :namespace_limit, inverse_of: :namespace
       has_one :gitlab_subscription, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
       has_one :elasticsearch_indexed_namespace
 
       accepts_nested_attributes_for :gitlab_subscription
+      accepts_nested_attributes_for :namespace_limit
 
       scope :include_gitlab_subscription, -> { includes(:gitlab_subscription) }
       scope :join_gitlab_subscription, -> { joins("LEFT OUTER JOIN gitlab_subscriptions ON gitlab_subscriptions.namespace_id=namespaces.id") }
@@ -53,6 +55,10 @@ module Namespace
       delegate :shared_runners_minutes, :shared_runners_seconds, :shared_runners_seconds_last_reset,
         :extra_shared_runners_minutes, to: :namespace_statistics, allow_nil: true
 
+      delegate :additional_purchased_storage_size, :additional_purchased_storage_size=,
+        :additional_purchased_storage_ends_on, :additional_purchased_storage_ends_on=,
+        to: :namespace_limit, allow_nil: true
+
       delegate :email, to: :owner, allow_nil: true, prefix: true
 
       # Opportunistically clear the +file_template_project_id+ if invalid
@@ -72,6 +78,10 @@ module Namespace
       before_save :clear_feature_available_cache
     end
 
+    def namespace_limit
+      super.presence || build_namespace_limit
+    end
+
     class_methods do
       extend ::Gitlab::Utils::Override