Add API client for notification statistic

Adds a client for the endpoints added in
67c4bd2263

app/notify_client/statistics_api_client.py

@@ -0,0 +1,18 @@
+from notifications_python_client.base import BaseAPIClient
+
+
+class StatisticsApiClient(BaseAPIClient):
+    def __init__(self, base_url=None, client_id=None, secret=None):
+        super(self.__class__, self).__init__(base_url=base_url or 'base_url',
+                                             client_id=client_id or 'client_id',
+                                             secret=secret or 'secret')
+
+    def init_app(self, app):
+        self.base_url = app.config['API_HOST_NAME']
+        self.client_id = app.config['ADMIN_CLIENT_USER_NAME']
+        self.secret = app.config['ADMIN_CLIENT_SECRET']
+
+    def get_statistics_for_service(self, service_id):
+        return self.get(
+            url='/service/{}/notifications-statistics'.format(service_id),
+        )