Implement async method for Disconnection Device

Refactor method `Disconnect` in `AstarteDevice`
to async `Disconnect`

Signed-off-by: Osman Hadzic <osman.hadzic@secomind.com>

AstarteDeviceSDKCSharpE2E.Tests/AstarteDeviceTest.cs

@@ -70,11 +70,11 @@ public void ConnectDeviceToAstarte()
     /// Test disconnect device from Astarte
     /// </summary>
     [Fact, TestPriority(9)]
-    public void DisconnectDeviceFromAstarte()
+    public async Task DisconnectDeviceFromAstarte()
     {
         Console.WriteLine("Test disconnect device to Astarte");
 
-        astarteDevice.Disconnect();
+        await astarteDevice.Disconnect();
         Assert.False(astarteDevice.IsConnected());
     }
 }