Create new gitlab_subscriptions internal API route

What does this MR do and why?

This MR creates a new internal API for gitlab_subscriptions resource to follow the new internal endpoint path format of: '/internal/gitlab_subscriptions/...'

It copies the existing code from API::GitlabSubscriptions::Subscriptions and its corresponding spec, almost verbatim.

The older endpoint will be deprecated and removed in #473625.

Related to Issue: #463741+

Corresponding MR on CDot: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/11122+

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.

How to set up and validate locally

• Start the GDK and CustomersDot.

• GitLab: Create a new top-level namespace that doesn't have gitlab_subscription

  namespace = Namespace.last
  namespace.gitlab_subscription.destroy
  namespace.id
  => 415

  CustomersDot: make the API request to create a GitLab subscription for that namespace

  # create a new subscription
  Client::GitlabApp.post(Gitlab::Api::Internal::Routes.subscription_path(415), **Gitlab::Api::Internal::Authentication.token, body: {start_date: Date.current, seats: 10, plan_code: 'premium' })
  
  # check that the subscription was created 
  Gitlab::Api::Internal::GitlabSubscription.find_by(gl_namespace_id: 415)
  
  # update the subscription seats to 20
  Client::GitlabApp.put(Gitlab::Api::Internal::Routes.subscription_path(415), **Gitlab::Api::Internal::Authentication.token, body: {seats: 20})
  
  # check that the subscription was updated 
  Gitlab::Api::Internal::GitlabSubscription.find_by(gl_namespace_id: 415)