Add ability to create a new group for subscription purchase

What does this MR do and why?

Add ability to create a new group for subscription purchase

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

⚠ ⚠ Cannot be merged until !157262 (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

Scenario	Screenshot
Group creation - Initial state
Group creation - validation error
Group name - validation error from API
Group URL preview - path available
Group URL preview - path unavailable, suggests unique path
Unable to suggest path error
Error while checking group path
Error while creating the group
When there are no eligible groups available
Group creation flow with redirection	Group creation.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