add extension_list example (#281)

- Created an example of extension policy
- Added C# binding support of .NET framework 4.0 and created a Nuget
  spec for it.
- Added a pytest in python bindings to test the extension policy and the
  python binding
- Restructured the example and Csharp binding directories due to above
  changes.
- Added copyrights.
- Added a Windows workflow for .NET 4.0 build and test.

examples/extension_list/agent-extension-input.json

@@ -0,0 +1,28 @@
+{
+    "extensions": {
+        "Microsoft.Azure.ActiveDirectory.AADSSHLoginForLinux": {
+            "signingInfo": {
+
+                "extensionSigned": false
+
+            }
+
+        },
+        "test2": {
+            "signingInfo": {
+                "extensionSigned": true
+            }
+        },
+        "test3": {
+            "signingInfo": {
+                "extensionSigned": false
+            }
+        },
+        "test1": {
+            "signingInfo": {
+                "extensionSigned": false
+            }
+        },
+        "test4": {}
+    }
+}