Add factories and shared contexts for V2 approval rules testing
Add FactoryBot factory and RSpec shared contexts to support testing v2 approval rules.
Requirements:
- Add FactoryBot factory for MergeRequests::ApprovalRule with origin: :merge_request
- Add traits: :with_users, :with_groups, :with_source_rule
Reference: v2 Approval Rules POC (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/222865)
issue