feat(cluster): add implicit route and gateway discovery via INFO gossip

Implements ProcessImplicitRoute and ForwardNewRouteInfoToKnownServers on RouteManager,
and ProcessImplicitGateway on GatewayManager, mirroring Go server/route.go and
server/gateway.go INFO gossip-based peer discovery. Adds ConnectUrls to ServerInfo
and introduces GatewayInfo model. 12 new unit tests in ImplicitDiscoveryTests.

src/NATS.Server/Protocol/NatsProtocol.cs

@@ -89,6 +89,10 @@ public sealed class ServerInfo
     [JsonPropertyName("tls_available")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
     public bool TlsAvailable { get; set; }
+
+    [JsonPropertyName("connect_urls")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string[]? ConnectUrls { get; set; }
 }
 
 public sealed class ClientOptions