Add Kubernetes authentication provider support

- Add KubernetesAuthProvider class for token validation using Kubernetes SelfSubjectReview API
- Add KubernetesAuthProviderConfig with configurable API server URL, TLS settings, and claims mapping
- Implement authentication via POST requests to /apis/authentication.k8s.io/v1/selfsubjectreviews endpoint
- Add support for parsing Kubernetes SelfSubjectReview response format to extract user information
- Add KUBERNETES provider type to AuthProviderType enum
- Update create_auth_provider factory function to handle 'kubernetes' provider type
- Add comprehensive unit tests for KubernetesAuthProvider functionality
- Add documentation with configuration examples and usage instructions

The provider validates tokens by sending SelfSubjectReview requests to the Kubernetes API server
and extracts user information from the userInfo structure in the response.

Signed-off-by: Akram Ben Aissi <akram.benaissi@gmail.com>

tests/unit/server/test_auth.py

@@ -774,3 +774,136 @@ def test_has_required_scope_function():
 
     # Test no user (auth disabled)
     assert _has_required_scope("test.read", None)
+
+
+@pytest.fixture
+def mock_kubernetes_api_server():
+    return "https://api.cluster.example.com:6443"
+
+
+@pytest.fixture
+def kubernetes_auth_app(mock_kubernetes_api_server):
+    app = FastAPI()
+    auth_config = AuthenticationConfig(
+        provider_config={
+            "type": "kubernetes",
+            "api_server_url": mock_kubernetes_api_server,
+            "verify_tls": False,
+            "claims_mapping": {
+                "username": "roles",
+                "groups": "roles",
+                "uid": "uid_attr",
+            },
+        },
+    )
+    app.add_middleware(AuthenticationMiddleware, auth_config=auth_config, impls={})
+
+    @app.get("/test")
+    def test_endpoint():
+        return {"message": "Authentication successful"}
+
+    return app
+
+
+@pytest.fixture
+def kubernetes_auth_client(kubernetes_auth_app):
+    return TestClient(kubernetes_auth_app)
+
+
+def test_missing_auth_header_kubernetes_auth(kubernetes_auth_client):
+    response = kubernetes_auth_client.get("/test")
+    assert response.status_code == 401
+    assert "Authentication required" in response.json()["error"]["message"]
+
+
+def test_invalid_auth_header_format_kubernetes_auth(kubernetes_auth_client):
+    response = kubernetes_auth_client.get("/test", headers={"Authorization": "InvalidFormat token123"})
+    assert response.status_code == 401
+    assert "Invalid Authorization header format" in response.json()["error"]["message"]
+
+
+async def mock_kubernetes_selfsubjectreview_success(*args, **kwargs):
+    return MockResponse(
+        201,
+        {
+            "apiVersion": "authentication.k8s.io/v1",
+            "kind": "SelfSubjectReview",
+            "metadata": {"creationTimestamp": "2025-07-15T13:53:56Z"},
+            "status": {
+                "userInfo": {
+                    "username": "alice",
+                    "uid": "alice-uid-123",
+                    "groups": ["system:authenticated", "developers", "admins"],
+                    "extra": {"scopes.authorization.openshift.io": ["user:full"]},
+                }
+            },
+        },
+    )
+
+
+async def mock_kubernetes_selfsubjectreview_failure(*args, **kwargs):
+    return MockResponse(401, {"message": "Unauthorized"})
+
+
+async def mock_kubernetes_selfsubjectreview_http_error(*args, **kwargs):
+    return MockResponse(500, {"message": "Internal Server Error"})
+
+
+@patch("httpx.AsyncClient.post", new=mock_kubernetes_selfsubjectreview_success)
+def test_valid_kubernetes_auth_authentication(kubernetes_auth_client, valid_token):
+    response = kubernetes_auth_client.get("/test", headers={"Authorization": f"Bearer {valid_token}"})
+    assert response.status_code == 200
+    assert response.json() == {"message": "Authentication successful"}
+
+
+@patch("httpx.AsyncClient.post", new=mock_kubernetes_selfsubjectreview_failure)
+def test_invalid_kubernetes_auth_authentication(kubernetes_auth_client, invalid_token):
+    response = kubernetes_auth_client.get("/test", headers={"Authorization": f"Bearer {invalid_token}"})
+    assert response.status_code == 401
+    assert "Invalid token" in response.json()["error"]["message"]
+
+
+@patch("httpx.AsyncClient.post", new=mock_kubernetes_selfsubjectreview_http_error)
+def test_kubernetes_auth_http_error(kubernetes_auth_client, valid_token):
+    response = kubernetes_auth_client.get("/test", headers={"Authorization": f"Bearer {valid_token}"})
+    assert response.status_code == 401
+    assert "Token validation failed" in response.json()["error"]["message"]
+
+
+def test_kubernetes_auth_request_payload(kubernetes_auth_client, valid_token, mock_kubernetes_api_server):
+    with patch("httpx.AsyncClient.post") as mock_post:
+        mock_response = MockResponse(
+            200,
+            {
+                "apiVersion": "authentication.k8s.io/v1",
+                "kind": "SelfSubjectReview",
+                "metadata": {"creationTimestamp": "2025-07-15T13:53:56Z"},
+                "status": {
+                    "userInfo": {
+                        "username": "test-user",
+                        "uid": "test-uid",
+                        "groups": ["test-group"],
+                    }
+                },
+            },
+        )
+        mock_post.return_value = mock_response
+
+        kubernetes_auth_client.get("/test", headers={"Authorization": f"Bearer {valid_token}"})
+
+        # Verify the request was made with correct parameters
+        mock_post.assert_called_once()
+        call_args = mock_post.call_args
+
+        # Check URL (passed as positional argument)
+        assert call_args[0][0] == f"{mock_kubernetes_api_server}/apis/authentication.k8s.io/v1/selfsubjectreviews"
+
+        # Check headers (passed as keyword argument)
+        headers = call_args[1]["headers"]
+        assert headers["Authorization"] == f"Bearer {valid_token}"
+        assert headers["Content-Type"] == "application/json"
+
+        # Check request body (passed as keyword argument)
+        request_body = call_args[1]["json"]
+        assert request_body["apiVersion"] == "authentication.k8s.io/v1"
+        assert request_body["kind"] == "SelfSubjectReview"