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/server/allowed_server.rego

@@ -0,0 +1,26 @@
+package example
+
+default allow := false                              # unless otherwise defined, allow is false
+
+allow := true {                                     # allow is true if...
+    count(violation) == 0                           # there are zero violations.
+}
+
+violation[server.id] {                              # a server is in the violation set if...
+    some server
+    public_server[server]                           # it exists in the 'public_server' set and...
+    server.protocols[_] == "http"                   # it contains the insecure "http" protocol.
+}
+
+violation[server.id] {                              # a server is in the violation set if...
+    server := input.servers[_]                      # it exists in the input.servers collection and...
+    server.protocols[_] == "telnet"                 # it contains the "telnet" protocol.
+}
+
+public_server[server] {                             # a server exists in the public_server set if...
+    some i, j
+    server := input.servers[_]                      # it exists in the input.servers collection and...
+    server.ports[_] == input.ports[i].id            # it references a port in the input.ports collection and...
+    input.ports[i].network == input.networks[j].id  # the port references a network in the input.networks collection and...
+    input.networks[j].public                        # the network is public.
+}

examples/server/input.json

@@ -0,0 +1,20 @@
+{
+    "servers": [
+        {"id": "app", "protocols": ["https", "ssh"], "ports": ["p1", "p2", "p3"]},
+        {"id": "db", "protocols": ["mysql"], "ports": ["p3"]},
+        {"id": "cache", "protocols": ["memcache"], "ports": ["p3"]},
+        {"id": "ci", "protocols": ["http"], "ports": ["p1", "p2"]},
+        {"id": "busybox", "protocols": ["telnet"], "ports": ["p1"]}
+    ],
+    "networks": [
+        {"id": "net1", "public": false},
+        {"id": "net2", "public": false},
+        {"id": "net3", "public": true},
+        {"id": "net4", "public": true}
+    ],
+    "ports": [
+        {"id": "p1", "network": "net1"},
+        {"id": "p2", "network": "net3"},
+        {"id": "p3", "network": "net2"}
+    ]
+}