Add ability to select a group for subscription purchase

What does this MR do and why?

Add ability to select a group for subscription purchase

• Adds a new component with group selection option
• Adds tests

This MR is the first of two MRs that implement the group creation page proposed here.

A follow-up MR will add the ability to create a new group on the same page. This MR will also have the logic to not show the group selector if no eligible groups exist but instead only show the new group creation controls.

⚠ ⚠ ⚠ Cannot be merged until !156854 (merged) is merged ⚠ ⚠ ⚠

Related to #460774

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.

Screenshots or screen recordings

Before	After
Group selection - Initial state
Group selection - validation error
Accordion with help text for groups
Ultimate subscription purchase flow - header with correct plan name
Group creation flow with redirection	Screen Recording 2024-06-25 at 6.15.52 PM.mov

How to set up and validate locally

1. Setup Customers Dot
2. Start GDK in SaaS mode GITLAB_SIMULATE_SAAS=1 gdk start
3. GitLab: Create groups
4. GitLab: Enable migrate_purchase_flows_for_existing_customers feature flag
5. GitLab: Navigate to http://localhost:3000/-/subscriptions/groups/new?plan_id=2c92a00d76f0d5060176f2fb0a5029ff for Premium plan
6. GitLab: Navigate to http://localhost:3000/-/subscriptions/groups/new?plan_id=2c92a0ff76f0d5250176f2f8c86f305a for Ultimate plan