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-default-data.json

@@ -0,0 +1,9 @@
+{
+    "azureGuestAgentPolicy": {
+        "policyVersion": "0.1.0",
+        "signingRules": {
+            "extensionSigned": false
+        },
+        "allowListOnly": false
+    }
+}