Create rate limiting window

lib/angel_security.dart

@@ -1 +1,2 @@
-export 'src/rate_limiter.dart';
+export 'src/rate_limiter.dart';
+export 'src/rate_limiting_window.dart';

lib/src/rate_limiting_window.dart

@@ -0,0 +1,15 @@
+/// A representation of the abstract "rate-limiting window" in which
+/// a [user] is accessing some API or endpoint.
+class RateLimitingWindow<User> {
+  /// The user who is accessing the endpoint.
+  final User user;
+
+  /// The time at which the user's current window began.
+  final DateTime start;
+
+  /// The number of requests the user has already sent within
+  /// the current window.
+  final int requestCount;
+
+  RateLimitingWindow(this.user, this.start, this.requestCount);
+}