Add IFileSystemHandler to support Azure for StaticMapping location (#180)

* wip

* CustomStaticMappingFileHandler

* Add unit-tests

* Tests

* IFileSystemHandler

* version

src/WireMock.Net/Handlers/IFileSystemHandler.cs

@@ -0,0 +1,49 @@
+using System.Collections.Generic;
+
+namespace WireMock.Handlers
+{
+    /// <summary>
+    /// Handler to interact with the file system to handle folders and read and write static mapping files.
+    /// </summary>
+    public interface IFileSystemHandler
+    {
+        /// <summary>
+        /// Gets the folder where the static mappings are located. For local file system, this would be `{CurrentFolder}/__admin/mappings`.
+        /// </summary>
+        /// <returns>The foldername.</returns>
+        string GetMappingFolder();
+
+        /// <summary>
+        /// Determines whether the given path refers to an existing directory on disk.
+        /// </summary>
+        /// <param name="path">The path.</param>
+        /// <returns>true if path refers to an existing directory; false if the directory does not exist or an error occurs when trying to determine if the specified directory exists.</returns>
+        bool FolderExists(string path);
+
+        /// <summary>
+        ///  Creates all directories and subdirectories in the specified path unless they already exist.
+        /// </summary>
+        /// <param name="path">The path.</param>
+        void CreateFolder(string path);
+
+        /// <summary>
+        ///  Returns an enumerable collection of file names in a specified path.
+        /// </summary>
+        /// <param name="path">The path.</param>
+        /// <returns>An enumerable collection of the full names (including paths) for the files in the directory specified by path.</returns>
+        IEnumerable<string> EnumerateFiles(string path);
+
+        /// <summary>
+        /// Read a static mapping file as text.
+        /// </summary>
+        /// <param name="path">The path (folder + filename with .json extension).</param>
+        string ReadMappingFile(string path);
+
+        /// <summary>
+        /// Write the static mapping.
+        /// </summary>
+        /// <param name="path">The path (folder + filename with .json extension).</param>
+        /// <param name="text">The text.</param>
+        void WriteMappingFile(string path, string text);
+    }
+}