Add a document download client

Allows uploading documents to the Document Download API.
The client is configured with an API host and auth token. There's
no need for a flag to disable the client in the test environments
at the moment since the upload is only triggered by a specific
payload which would only be sent with an explicit goal of using
document download.

tests/app/clients/test_document_download.py

@@ -0,0 +1,37 @@
+import requests
+import requests_mock
+import pytest
+
+from app.clients.document_download import DocumentDownloadClient
+
+
+@pytest.fixture(scope='function')
+def document_download(client, mocker):
+    client = DocumentDownloadClient()
+    current_app = mocker.Mock(config={
+        'DOCUMENT_DOWNLOAD_API_HOST': 'https://document-download',
+        'DOCUMENT_DOWNLOAD_API_KEY': 'test-key'
+    })
+    client.init_app(current_app)
+    return client
+
+
+def test_get_upload_url(document_download):
+    assert document_download.get_upload_url('service-id') == 'https://document-download/services/service-id/documents'
+
+
+def test_upload_document(document_download):
+    with requests_mock.Mocker() as request_mock:
+        request_mock.post('https://document-download/services/service-id/documents', json={
+            'document': {'url': 'https://document-download/services/service-id/documents/uploaded-url'}
+        }, status_code=201)
+
+        resp = document_download.upload_document('service-id', 'abababab')
+
+    assert resp == 'https://document-download/services/service-id/documents/uploaded-url'
+
+
+def test_should_raise_for_status(document_download):
+    with pytest.raises(requests.HTTPError), requests_mock.Mocker() as request_mock:
+        request_mock.post('https://document-download/services/service-id/documents', json={}, status_code=403)
+        document_download.upload_document('service-id', 'abababab')