Add cart abandonment modal

What does this MR do and why?

We believe that we can capture and direct pricing interest towards two valuable activities: talking to Sales (PQL) and trial upgrade (key conversion flow).

To verify that, we will provide a CTA to talk to Sales or start a free Ultimate trial when a user exits the SaaS purchase flow before completion.

And we’ll measure the impact on PQL count and free-to-trial conversion rate.

Screenshots or screen recordings

How to set up and validate locally

1. Enable cart_abandonment_modal feature flag.
2. Login as any user.
3. Go to http://localhost:3000/-/subscriptions/new
4. Wait 3 minutes. Modal window will appear

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/345177