Implement create service for v2 approval rules

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Implement create service for v2 approval rules.

The service should:

• Check authorization and validation
• Create MergeRequests::ApprovalRule with:
  • origin: :merge_request
  • project_id from merge request's target project
  • Default approvals_required: 0 if not specified
  • Default rule_type: :regular if not specified
• Associate rule with merge request via MergeRequests::ApprovalRulesMergeRequest join table
• Assign user approvers (if user_ids provided)
• Assign group approvers (if group_ids provided)
• Return ServiceResponse with success/error

Reference: POC implementation in poc/v2-approval-rules-complete branch.