Issue 4954 - Implement a downloader script supporting OAuth2 authentication/authorization

sitescripts/oauth2dl/README.md

Index: sitescripts/oauth2dl/README.md
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+# oauth2dl 
+
+This is a script that downloads a file from a given URL 
+using Google Oauth2 for service accounts.
+
+## Dependencies
+
+* [Python 2.7](https://www.python.org/download/releases/2.7/)
+* [oauth2client](https://github.com/google/oauth2client) Using pip:
+```commandline
+    pip install oauth2client
+```
+* [httplib2](https://github.com/httplib2/httplib2) Using pip:
+```commandline
+    pip install httplib2
+```
+    
+## Running the script
+
+The script can be run as a module, using the following command:
+
+```commandline
+    python -m sitescripts.oauth2dl.bin.oauth2dl [-k <key-file>] [-s <scope>] [-o <path>] <url>
+```
+
+where: 
+* `key-file` = Path to the key file used to authenticate. If not provided, uses
+the `OAUTH2DL_KEY` environment variable.
+* `scope` = The scope used when authenticating(eg:
+https://www.googleapis.com/auth/drive). If not provided, uses the `OAUTH2DL_SCOPE`
+environment variable.
+
+* `path` = Path where to save the downloaded file. If not provided, the 
+contents will be sent to `stdout`
+* `url` = URL of the file we're trying to download. For a Google Drive 
+file, the url should look something like:  
+`https://www.googleapis.com/drive/v3/files/<fileID>?alt=media`
+
+The script can also be run directly from the file, using:
+```commandline
+    python sitescripts/oauth2dl/bin/oauth2dl.py [-k <key-file>] [-s <scope>] [-o <path>] <url>
+```
+
+## Getting the key file
+
+The key file can be obtained by following the instructions available 
+[here](https://developers.google.com/identity/protocols/OAuth2ServiceAccount).
+
+It should be a JSON with the following format: 
+```json
+    {
+      "type": "service_account",
+      "project_id": <project_id>,
+      "private_key_id": <private_key_id>,
+      "private_key": <RSA-encrypted private key>,
+      "client_email": "<service_account_id>@<project_id>.iam.gserviceaccount.com",
+      "client_id": <client_id>,
+      "auth_uri": "https://accounts.google.com/o/oauth2/auth",
+      "token_uri": "https://accounts.google.com/o/oauth2/token",
+      "auth_provider_x509_cert_url": ...,
+      "client_x509_cert_url": ...
+}
+```
+
+In order to download the file the *service account* (i.e. the email in 
+the `client_id` field from the key file) should be granted access to it.
+
+