BE: Add data to list pending promotion members on Admin Page

Objective

Objective: create the BE list of user pending promotion that will be surfaced to the admin for approval

Scope: self-managed GitLab Ultimate

Work to be done

Summary of work	Description	MR
Listing the pending promotion users for the Admin to Approve in Admin Area	The Backend changes which will integrate with the new UI tab and provide the list of data that would be used to display the pending promotion users.

Open Items

• FE / BE Alignment: determine what the Backend should return as response, which will then be consumed by the frontend to display the "Pending Promotion" tab. Something similar to this MR which discusses the Backend/Frontend handshake to display the "Pending Promotion" on the Group and Project Members page.
• Re-usability: determine if changes made as part of the Project/Group member listing (structure of backend response and such) can be reused here

Original Issue Contents

List Pending Promotion requests.

1. Identify how data will be shown on FE
2. Make BE changes to return data for the same