Add API client for notifications

This commit adds an API client for the notifications endpoints added in:
https://github.com/alphagov/notifications-api/pull/113

tests/app/main/notify_client/test_notification_client.py

@@ -0,0 +1,41 @@
+import pytest
+from app.notify_client.notification_api_client import NotificationApiClient
+
+
+def test_client_gets_notifications(mocker):
+
+    mock_get = mocker.patch('app.notify_client.notification_api_client.NotificationApiClient.get')
+    NotificationApiClient().get_all_notifications()
+    mock_get.assert_called_once_with(url='/notifications', params={})
+
+
+def test_client_gets_notifications_with_page(mocker):
+
+    mock_get = mocker.patch('app.notify_client.notification_api_client.NotificationApiClient.get')
+    NotificationApiClient().get_all_notifications(page=99)
+    mock_get.assert_called_once_with(url='/notifications', params={'page': 99})
+
+
+@pytest.mark.parametrize("arguments,expected_call", [
+    (
+        {},
+        {'url': '/service/abcd1234/notifications', 'params': {}}
+    ),
+    (
+        {'page': 99},
+        {'url': '/service/abcd1234/notifications', 'params': {'page': 99}}
+    ),
+    (
+        {'job_id': 'efgh5678'},
+        {'url': '/service/abcd1234/job/efgh5678/notifications', 'params': {}}
+    ),
+    (
+        {'job_id': 'efgh5678', 'page': 48},
+        {'url': '/service/abcd1234/job/efgh5678/notifications', 'params': {'page': 48}}
+    )
+])
+def test_client_gets_notifications_for_service_and_job_by_page(mocker, arguments, expected_call):
+
+    mock_get = mocker.patch('app.notify_client.notification_api_client.NotificationApiClient.get')
+    NotificationApiClient().get_notifications_for_service('abcd1234', **arguments)
+    mock_get.assert_called_once_with(**expected_call)