GenServer-based gRPC connection worker with automatic reconnection.

Each worker manages a single gRPC connection and stores its channel directly in the pool's ETS table for zero-GenServer-call access from Pool.get_channel/1.

Features:

• Channels stored in ETS for O(1) pool access (no GenServer.call in hot path)
• Automatic reconnection with exponential backoff and jitter
• Active disconnect detection by monitoring the gRPC connection process
• Self-registration in Registry for health tracking
• Optional periodic ping to keep connections warm
• Configurable max reconnect attempts before crash

Disconnect detection (grpc 1.0)

grpc 1.0's Gun adapter owns the socket inside its own supervised GRPC.Client.Adapters.Gun.ConnectionProcess, so gun's :gun_down/:gun_error messages no longer reach this worker. Worse, when the connection drops the inner gun process dies but the ConnectionProcess (the conn_pid in channel.adapter_payload) lingers as a zombie — so monitoring conn_pid cannot detect a drop.

Instead we monitor the inner gun process, read out of the adapter's ConnectionProcess state, and pair it with adapter_opts: [retry: 0] (see GrpcConnectionPool.Config) so gun gives up immediately on a drop and this pool's own Backoff governs reconnection. Reaching into the adapter state couples us to a grpc internal; it is guarded and falls back to monitoring conn_pid if the state shape changes.