NSP Target Registry chained server

[LionelJouin]

Description

• New target registry server pattern allowing future feature, e.g.:
  • Logger / Metrics / Trace
  • KeepAlive
  • NSP multi-replicas support
• Current chain: watch handler -> store (sqlite + keepalive)

note: if this PR get accepted I will implement the same pattern for other APIs (TAPA, IPAM, NSP Configuration manager)

Issue link

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Checklist

• Purpose
  • [ ] Bug fix
  • [ ] New functionality
  • [ ] Documentation
  • [x] Refactoring
  • [ ] CI
• Test
  • [x] Unit test
  • [ ] E2E Test
  • [x] Tested manually
• Introduce a breaking change
  • [ ] Yes (description required)
  • [x] No
• Introduce changes in the Operator
  • [ ] Yes (description required)
  • [x] No