Extract subscription users table into its own component

What does this MR do and why?

Extracts subscription users table on Usage quotas > Seats page into its own component to reduce complexity in subscription_seats.vue component especially when we add more functionality to the users table. There is no change to the existing logic and is pretty much about moving the table out of subscription_seats.vue into a new component, similar changes in test file too.

Related to #411949

Screenshots or screen recordings

No changes to UI, before and after should be identical

Scenario	Before	After
Free plan	Screenshot	Screenshot
Premium plan (usage quota page includes guest members)	Screenshot	Screenshot
Ultimate plan (usage quota page does not include guest members)	Screenshot	Screenshot
Sort functionality	Screenshot	Screenshot
User details	Screenshot	Screenshot
Export	Screenshot Export	Screenshot Export
Filter - no users to display	Screenshot	Screenshot
Filter - minimum 3 characters required	Screenshot	Screenshot
Remove user	Screenshot Remove_user	Screenshot Remove_user
Cannot remove	Screenshot	Screenshot

How to set up and validate locally

1. Customers Dot needs to be setup in order to view the billing page for a group
2. Start GDK in SaaS mode GITLAB_SIMULATE_SAAS=1 gdk start
3. Create a Group
4. Buy a Subscription for your group from Settings > Billing
5. Visit /groups/<your-namespace-name>/-/usage_quotas#seats-quota-tab

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.