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.

.github/workflows/test-csharp.yml

@@ -19,8 +19,8 @@ jobs:
 
       - name: Build
         run: dotnet build
-        working-directory: ./bindings/csharp
+        working-directory: ./bindings/csharp/net8.0
 
       - name: Run
         run: LD_LIBRARY_PATH=. dotnet run
-        working-directory: ./bindings/csharp
+        working-directory: ./bindings/csharp/net8.0

.github/workflows/test-csharp40.yml

@@ -0,0 +1,28 @@
+name: bindings/csharp40
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  test:
+    runs-on: windows-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: actions/setup-dotnet@v1
+        with:
+            dotnet-version: "5.0.x"
+
+      - name: Build
+        run: dotnet build
+        working-directory: ./bindings/csharp/net40
+
+      - name: Run
+        run: dotnet run
+        working-directory: ./bindings/csharp/net40