Add TimeSettings (Start, End and TTL) (#661)

* time

* .

* UT

* using JetBrains.Annotations;

src/WireMock.Net/Models/TimeSettings.cs

@@ -0,0 +1,19 @@
+using System;
+
+namespace WireMock.Models
+{
+    /// <summary>
+    /// TimeSettingsModel: Start, End and TTL
+    /// </summary>
+    public class TimeSettings : ITimeSettings
+    {
+        /// <inheritdoc />
+        public DateTime? Start { get; set; }
+
+        /// <inheritdoc />
+        public DateTime? End { get; set; }
+
+        /// <inheritdoc />
+        public int? TTL { get; set; }
+    }
+}