Add API client for notification statistic

Adds a client for the endpoints added in
67c4bd2263

tests/app/main/notify_client/test_statistics_client.py

@@ -0,0 +1,13 @@
+from app.notify_client.statistics_api_client import StatisticsApiClient
+
+
+def test_client_uses_correct_find_by_email(mocker, api_user_active):
+
+    expected_url = '/service/a1b2c3d4/notifications-statistics'
+
+    client = StatisticsApiClient()
+    mock_get = mocker.patch('app.notify_client.statistics_api_client.StatisticsApiClient.get')
+
+    client.get_statistics_for_service('a1b2c3d4')
+
+    mock_get.assert_called_once_with(url=expected_url)