feat: add gateway and DNS fields to pool model (#32)

[mgazza]

Summary

• Add optional Gateway (string) and DNS ([]string) fields to store.Pool model with json/cbor tags
• Add ValidateGateway() and ValidateDNS() validation functions: gateway must be a valid IP or empty, each DNS entry must be a valid IP
• Update PoolRequest, PoolResponse, createPool handler, and poolToResponse to support the new fields
• Fields are optional for backwards compatibility — empty gateway/dns means "not configured" (BNG falls back to local flags)

Test plan

• All existing tests pass (go test ./... -count=1)
• Verify pool creation with gateway and DNS fields via API
• Verify pool creation without gateway/DNS still works (backwards compatible)
• Verify invalid gateway/DNS values are rejected with 400

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