Add patroni_proxy - high-performance TCP proxy for Patroni clusters

[vadv]

Summary

This PR introduces patroni_proxy — a specialized high-performance TCP proxy for Patroni-managed PostgreSQL clusters. Following the Unix philosophy of "do one thing and do it well", it focuses exclusively on TCP load balancing and failover without disrupting existing connections during topology changes.

Key Features

• Zero-downtime connection management — existing connections are preserved during cluster topology changes (unlike HAProxy)
• Hot upstream updates — automatic discovery of cluster members via Patroni REST API (/cluster endpoint)
• Role-based routing — route connections to leader, sync replicas, or async replicas
• Least Connections load balancing — with connection counter preservation during updates
• Replication lag awareness — configurable max_lag_in_bytes with automatic client disconnection
• Member state filtering — only members with state: "running" are used as backends
• Configuration hot-reload — via SIGHUP signal without restart