Update billable_members/memberships endpoint to include invited group

Proposal

We need to update the GET /groups/:id/billable_members/:user_id/memberships endpoint to include the invited group(s) of the user.

This will unblock the work in #351954 (closed).

Availability and Testing

• No new E2E tests or updates needed at this time
• Consider adding request specs for the billable_members endpoint to verify the new expected details are returned for members from invited groups

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Support Priority Score: (-, -, -, -, -, -, -, -, -, -, -) => 3