Create base service for v2 approval rules

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Create base service for v2 approval rules.

Requirements:

• Shared authorisation helper (can_update_approvers?)
• ServiceResponse helpers (error/success)
• Accessors (merge_request, current_user, params, project)

This will be used by the new v2 Create, Update, and Destroy services.

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