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.

.gitignore

@@ -1,6 +1,8 @@
 # Generated by Cargo
 # will have compiled files and executables
 /target/
+**/wheels/
+**/__pycache__/
 
 # Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
 # More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html