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

src/NATS.Server/Auth/AuthResult.cs

@@ -0,0 +1,9 @@
+namespace NATS.Server.Auth;
+
+public sealed class AuthResult
+{
+    public required string Identity { get; init; }
+    public string? AccountName { get; init; }
+    public Permissions? Permissions { get; init; }
+    public DateTimeOffset? Expiry { get; init; }
+}