supernode/p2p: cache validator lookups to cut handshake gRPC reads

[mateeullahmalik]

• Add 1h TTL cache (Ristretto + singleflight) in validator.go for:
  • Auth.AccountInfoByAddress
  • SuperNode.GetSupernodeBySupernodeAddress
• Impact on p2p handshakes:
  • Before: ~2 Lumera gRPC reads per connection (one per side) on every handshake.
  • After: ≤1 read per side per identity per hour; subsequent handshakes are cache hits; concurrent misses collapsed to 1.
• Scope: pkg/lumera/validator.go only.
• Risk: up to 1h staleness (acceptable for identity/supernode registry).