Add WebsocketConfig to ServerConfig so the embedded nats-server can expose an
additional WebSocket port (nats.ws) alongside the TCP data plane. This lets a
browser SPA speak the NATS protocol directly, the way native TCP peers (Go,
Kotlin/android) already do — the first enabler for uniweb becoming a
browser-native client with no Go gateway (issue uniweb/0001, Phase 0).
The client authenticator applies to WebSocket connections too, so this adds a
transport, not a trust bypass. Plain ws:// is used only without TLS (loopback
dev); a certificate yields wss://. An empty AllowedOrigins enforces same-origin.
Nil WebsocketConfig keeps the server TCP-only, so existing single-node and
cluster deployments are unchanged.
Tests: WebSocket listener opens and completes the upgrade handshake (101); no
listener opens when WebsocketConfig is nil.
egutierrez
36f4ba0eaf