Issue #91: Added MarketBookGetRequest to perform function MarketBookGet (MT5)

MtApi5/Requests/MarketBookGetRequest.cs

@@ -0,0 +1,9 @@
+namespace MtApi5.Requests
+{
+    internal class MarketBookGetRequest: RequestBase
+    {
+        public override RequestType RequestType => RequestType.MarketBookGet;
+
+        public string Symbol { get; set; }
+    }
+}

MtApi5/Requests/RequestType.cs

@@ -9,6 +9,7 @@ namespace MtApi5.Requests
         iCustom         = 2,
         OrderSend       = 3,
         PositionOpen    = 4,
-        OrderCheck      = 5
+        OrderCheck      = 5,
+        MarketBookGet   = 6
     }
 }