feat: Add response delay generator to mock_server

packages/mock_server/lib/response_delay_generator.dart

@@ -0,0 +1,46 @@
+import 'dart:math';
+
+abstract interface class DelayGenerator {
+  Duration nextDelay();
+}
+
+class RandomDelayGenerator implements DelayGenerator {
+  /// Minimum allowed response delay
+  final Duration minDelay;
+
+  /// Maximum allowed response delay
+  final Duration maxDelay;
+
+  final Random _random = Random();
+  RandomDelayGenerator(this.minDelay, this.maxDelay);
+
+  @override
+  Duration nextDelay() {
+    return Duration(
+      milliseconds: minDelay.inMilliseconds +
+          _random.nextInt(
+            maxDelay.inMilliseconds - minDelay.inMilliseconds,
+          ),
+    );
+  }
+}
+
+class ConstantDelayGenerator implements DelayGenerator {
+  final Duration delay;
+
+  const ConstantDelayGenerator(this.delay);
+
+  @override
+  Duration nextDelay() {
+    return delay;
+  }
+}
+
+class ZeroDelayGenerator implements DelayGenerator {
+  const ZeroDelayGenerator();
+
+  @override
+  Duration nextDelay() {
+    return Duration.zero;
+  }
+}