Add pending_members route

What does this MR do and why?

It adds a new route for listing pending members behind saas_user_caps feature flag.

Note: the view currently will only show an empty page (Vue code will be added in upcoming MR), the feature flag is off so merging this won't affect the users.

Merge request	Status
Add pending_members route	👈 you're here
Add alert to seats usage if there are any pending members	⏳ in review
Show pending members in Seats Usage -> Pending Members	⌛

Screenshots or screen recordings

How to set up and validate locally

1. Enable saas_user_caps feature flag from rails console. (rails console then Feature.enable(:saas_user_caps))
2. Navigate to a group with a paid plan (e.g. Ultimate)
3. Manually navigate to pending_members route (e.g. http://localhost:3000/groups/GROUP_ID/-/usage_quotas/pending_members)
4. You should see the empty page (only titles visible).

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 #333064 (closed)