Add Billable Memberships API

What does this MR do?

Add a Billable Memberships API - /groups/:id/billable_members/:user_id/memberships.

Returns all the user's direct memberships that fall within the group's hierarchy. This will not include inherited memberships.

Builds on the billable members API: https://docs.gitlab.com/ee/api/members.html#list-all-billable-members-of-a-group

Follows the design of this similar admin-only API for user memberships: https://docs.gitlab.com/ee/api/users.html#user-memberships-admin-only

For use in #321560 (closed)

Does this MR meet the acceptance criteria?

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