feat: add Subscription types and subject validation with wildcard matching

src/NATS.Server/Subscriptions/Subscription.cs

@@ -0,0 +1,10 @@
+namespace NATS.Server.Subscriptions;
+
+public sealed class Subscription
+{
+    public required string Subject { get; init; }
+    public string? Queue { get; init; }
+    public required string Sid { get; init; }
+    public long MessageCount;  // Interlocked
+    public long MaxMessages;   // 0 = unlimited
+}