feat: add INatsClient interface and implement on NatsClient

Extract INatsClient interface from NatsClient to enable internal clients
(SYSTEM, ACCOUNT) to participate in the subscription system without
requiring a socket connection. Change Subscription.Client from concrete
NatsClient to INatsClient, keeping IMessageRouter and RemoveClient using
the concrete type since only socket clients need those paths.

src/NATS.Server/INatsClient.cs

@@ -0,0 +1,19 @@
+using NATS.Server.Auth;
+using NATS.Server.Protocol;
+
+namespace NATS.Server;
+
+public interface INatsClient
+{
+    ulong Id { get; }
+    ClientKind Kind { get; }
+    bool IsInternal => Kind.IsInternal();
+    Account? Account { get; }
+    ClientOptions? ClientOpts { get; }
+    ClientPermissions? Permissions { get; }
+
+    void SendMessage(string subject, string sid, string? replyTo,
+        ReadOnlyMemory<byte> headers, ReadOnlyMemory<byte> payload);
+    bool QueueOutbound(ReadOnlyMemory<byte> data);
+    void RemoveSubscription(string sid);
+}