Check checksum for plugin downloads

* Compare the MD5 checksum when downloading plugins
* Reduced log spam due to http requests
* Removed 'GetTempFileResponse' function from HttpClientManager
* Fixed caching for HttpClientManager

MediaBrowser.Common/Extensions/HexHelper.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Globalization;
+
+namespace MediaBrowser.Common.Extensions
+{
+    public static class HexHelper
+    {
+        public static byte[] FromHexString(string str)
+        {
+            byte[] bytes = new byte[str.Length / 2];
+            for (int i = 0; i < str.Length; i += 2)
+            {
+                bytes[i / 2] = byte.Parse(str.Substring(i, 2), NumberStyles.HexNumber, CultureInfo.InvariantCulture);
+            }
+
+            return bytes;
+        }
+
+        public static string ToHexString(byte[] bytes)
+            => BitConverter.ToString(bytes).Replace("-", "");
+    }
+}