Integrate group level to project level MR approval rules

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Why are we doing this work

Users would like to integrate group-level MR approval settings into children projects.

Relevant links

• See #293957 for high-level design.
• Configure project approval rule via UI

Functional requirements

• Allow retrieving group-level approval rules via project-level approval rules API
  • Public endpoint GET /api/:version/project/:id/approval_rules
  • Public endpoint GET /api/:version/project/:id/approval_rules/:approval_rule_id
  • Internal endpoint GET /api/:version/project/:id/approval_settings
• Group-level approval rules should display on Project > Settings > Merge request approvals
• User can edit/delete group-level approval rules

Non-functional requirements

• [-] Documentation: Not needed as it is behind a feature flag
• Feature flag: group_merge_request_approval_settings_feature_flag
• [-] Performance:
• Testing: Ensure that the project approval rules works with feature flag/licensed disabled

Implementation plan

• backend Extend ApprovalState to support group (ie. user_defined_rules should account for group approval rules)

graph LR
  API::ProjectApprovalRules --> Project --> ApprovalState
  ApprovalState --> ApprovalProjectRule
  ApprovalState --> ApprovalGroupRule