Expose billable members membership_state

What does this MR do and why?

This exposes the state of a member when queried via the billable_member API which is required for rendering the toggle in https://gitlab.com/gitlab-org/gitlab/-/issues/352638+

Screenshots or screen recordings

How to set up and validate locally

1. Go to http://localhost:3000/groups/testing-subscription-over-group/-/usage_quotas#seats-quota-tab and look at the network tab which is now loading membership_state

or validate via curl

curl --request GET --header "PRIVATE-TOKEN: TOKEN" "http://localhost:3000/api/v4/groups/GROUP_ID/billable_members" | json_pp

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.