Experiment: Add Billing to a top-level navigation item

What does this MR do and why?

• Adds Billing menu item in the side nav as an experiment based on the qualifications in https://gitlab.com/gitlab-org/gitlab/-/issues/338540 and flow show in https://gitlab.com/groups/gitlab-org/-/epics/5954#flow

Screenshots or screen recordings

In side bar

expanded	collapsed

after clicking

expanded	collapsed

Mobile

How to set up and validate locally

1. Turn on billing features in the app(via rails console)

   ApplicationSetting.first.update(check_namespace_plan: true)

2. Enable the experiment fully(via rails console)

   Feature.enable(:billing_in_side_nav)

3. Visit any group or project pages such as http://localhost:3000/flightjs
4. Observe the Billings link in the side nav
5. Click link and see that your are redirected to the top level group's billing page with correct highlighting.

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 #338540