Add product_analytics_enabled application setting

What does this MR do and why?

• Adds an application setting to enable or disable the new product analytics stack currently in development.
  • The frontend work will be completed in #375577 (closed).

How to set up and validate locally

• Make sure the migration in this MR has been executed.
• Enable the feature flag (we're not testing that here): Feature.enable(:cube_api_proxy)
• Attempt to initialize the analytics stack for a project:

  ProductAnalytics::InitializeStackService.new(container: Project.first, current_user: User.first).execute

• Validate that an instance of ProductAnalytics::InitializeAnalyticsWorker was not enqueued in sidekiq.
• Enable the newly created application setting: Gitlab::CurrentSettings.update!(product_analytics_enabled: true)
• Attempt to initialize the analytics stack for a project:

  ProductAnalytics::InitializeStackService.new(container: Project.first, current_user: User.first).execute

• Validate that an instance of ProductAnalytics::InitializeAnalyticsWorker was enqueued in sidekiq. ✅

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #376678 (closed)