refactor: introduce policy-based config schema

[ktrubilo9]

Summary

• moves config to server/load_balancing/security.policies structure
• replaces backend URL references with backend IDs
• adds security_policy_id for virtual hosts
• regenerates adminpb from updated admin.proto
• updates admin gRPC/HTTP handlers and example configs

Main changes

• old root-level proxy_port and lb_strategy are removed
• virtual hosts now use backends_ids instead of backend URLs
• per-vhost inline security config is replaced by reusable security policies

Verification

• go test ./...
• go vet ./...