Add ability to track when MR-level rules override project-level rules

Tracks which project-level rule an MR-level rule was derived from.

• Adds "Reset to project defaults" functionality
• Showing users when they've customised a rule
• Security policy synchronization (reset_required_approvals method)

Requirements (validate these during implementation and adjust as needed):

• Add to MergeRequests::ApprovalRule model:
  • Add belongs_to :source_rule association (self-referential, optional)
    • class_name: 'MergeRequests::ApprovalRule'
    • optional: true
  • Implement overridden? method
    • Returns true if source_rule.present?
  • Update hook_attrs method to include:
    • modified_from_project_rule: overridden?

Reference: v2 Approval Rules POC