Extend namespace gitlab_subscription API

What does this MR do and why?

Add exclude_guests boolean field to the GET /namespaces/:id/gitlab_subscription endpoint. We will late consume this API on FE side to show overage modal.

Step	MR
Extend API with exclude_guests	👈 you are here
Fetch this API to conditionally show overage modal when adding members/groups to a group	follow-up MR

Related to #353455 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Reach the http://localhost:3000/api/v4/namespaces/<YOUR_NAMESPACE_ID>/gitlab_subscription
2. The "plans" should have an extra exclude_guests field

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.