Added namespace actor to Product Analytics billing feature flag

What does this MR do and why?

This MR is in continuation with https://gitlab.com/gitlab-org/gitlab/-/issues/438399 where we are enabling product analytics billing.

This adds a namespace actor to product_analytics_billing feature flag. Namespaces with the flag enabled will be able to purchase product_analytics addon and use product analytics features.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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How to set up and validate locally

• Set GDK to SaaS simulation and enable product analytics feature flag.

• Create a new group, with ultimate subscription.

• Try to purchase product_analytics addon for the group namespace.

  POST /api/v4/namespaces/{namespace_id}/subscription_add_on_purchase/product_analytics
  
  Request Body - {"quantity":"1","expires_on":"2024-03-03","purchase_xid":"1234"}

• You should get a 500 error. The addon cannot be purchased as the billing flag product_analytics_billing is not enabled for the namespace.

• You can also check the status of billing flag for a given namespace using

  Feature.enabled?(:product_analytics_billing, namespace, type: :wip)

• Now enable the feature flag product_analytics_billing for the group namespace

  Feature.enable(:product_analytics_billing, namespace, type: :wip)

• Try to purchase the product_analytics addon again. You should get a 200 OK response this time.

• Verify that addon and addon purchase are assigned to the namespace

  [3] pry(main)> GitlabSubscriptions::AddOnPurchase.last
  => #<GitlabSubscriptions::AddOnPurchase:0x000000012a1777c8
  id: 1,
  created_at: Fri, 23 Feb 2024 07:38:18.079237000 UTC +00:00,
  updated_at: Fri, 23 Feb 2024 07:38:18.079237000 UTC +00:00,
  subscription_add_on_id: 1,
  namespace_id: 24,
  quantity: 1,
  expires_on: Wed, 01 Jan 2025,
  purchase_xid: "ANY STRING",
  last_assigned_users_refreshed_at: nil>
  
  [4] pry(main)> GitlabSubscriptions::AddOn.last
  => #<GitlabSubscriptions::AddOn:0x000000012a673948
  id: 1,
  created_at: Fri, 23 Feb 2024 07:38:18.043147000 UTC +00:00,
  updated_at: Fri, 23 Feb 2024 07:38:18.043147000 UTC +00:00,
  name: "product_analytics",
  description: "[FILTERED]">

Related to #441245