feat: add IAuthenticator interface and TokenAuthenticator with constant-time comparison

src/NATS.Server/Auth/IAuthenticator.cs

@@ -0,0 +1,14 @@
+using NATS.Server.Protocol;
+
+namespace NATS.Server.Auth;
+
+public interface IAuthenticator
+{
+    AuthResult? Authenticate(ClientAuthContext context);
+}
+
+public sealed class ClientAuthContext
+{
+    public required ClientOptions Opts { get; init; }
+    public required byte[] Nonce { get; init; }
+}