Added new table MemberApprovals and corresponding associations

What does this MR do and why?

This Merge Request introduces a new table for managing member additions and promotion within groups and projects. The changes include the creation of a new MemberApproval model and its associated database table, along with necessary associations and foreign keys to integrate it seamlessly with our existing models: Member, User, Project models.

Key Changes:

1. New Table Creation - member_approvals:
   • The member_approvals table is added to store approval requests for new members. It includes fields for tracking the member, source (group or project), the user who requested the addition, the admin who approved it, and the approval status and time for approval/rejection.
2. Model Integration:
   • Associations are established between MemberApproval and other models (Member, User, Project) to ensure coherent data flow and integrity.

ref: https://gitlab.com/gitlab-org/gitlab/-/issues/433171

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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