feat: expose groups and vpns related list APIs

[rahmatrhd]

New APIs

• list groups /api/v1/groups?page=1&per_page=10
• list group vpns /api/v1/groups/:id/vpns
• list group admins /api/v1/groups/:id/admins
• list vpns /api/v1/vpns?page=1&per_page=10
• list vpn groups /api/v1/vpns/:id/groups