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

* wip

* CustomStaticMappingFileHandler

* Add unit-tests

* Tests

* IFileSystemHandler

* version

src/WireMock.Net/Handlers/LocalFileSystemHandler.cs

@@ -0,0 +1,49 @@
+using System.Collections.Generic;
+using System.IO;
+
+namespace WireMock.Handlers
+{
+    /// <summary>
+    /// Default implementation for a handler to interact with the local file system to read and write static mapping files.
+    /// </summary>
+    public class LocalFileSystemHandler : IFileSystemHandler
+    {
+        private static readonly string AdminMappingsFolder = Path.Combine("__admin", "mappings");
+
+        /// <inheritdoc cref="IFileSystemHandler.FolderExists"/>
+        public bool FolderExists(string path)
+        {
+            return Directory.Exists(path);
+        }
+
+        /// <inheritdoc cref="IFileSystemHandler.CreateFolder"/>
+        public void CreateFolder(string path)
+        {
+            Directory.CreateDirectory(path);
+        }
+
+        /// <inheritdoc cref="IFileSystemHandler.EnumerateFiles"/>
+        public IEnumerable<string> EnumerateFiles(string path)
+        {
+            return Directory.EnumerateFiles(path);
+        }
+
+        /// <inheritdoc cref="IFileSystemHandler.GetMappingFolder"/>
+        public string GetMappingFolder()
+        {
+            return Path.Combine(Directory.GetCurrentDirectory(), AdminMappingsFolder);
+        }
+
+        /// <inheritdoc cref="IFileSystemHandler.ReadMappingFile"/>
+        public string ReadMappingFile(string path)
+        {
+            return File.ReadAllText(path);
+        }
+
+        /// <inheritdoc cref="IFileSystemHandler.WriteMappingFile"/>
+        public void WriteMappingFile(string path, string text)
+        {
+            File.WriteAllText(path, text);
+        }
+    }
+}