Allow passing google auth Credentials directly to GCS connector

Among other things, this enables authenticating the GCS connector
using gcloud oauth from the parent shell, rather than requiring the
use of service account credentials.

parsons/google/google_cloud_storage.py

@@ -1,14 +1,15 @@
 import datetime
 import gzip
-import petl
 import logging
 import time
 import uuid
 import zipfile
-from typing import Optional
+from typing import Optional, Union
 
 import google
+import petl
 from google.cloud import storage, storage_transfer
+from google.oauth2.credentials import Credentials
 
 from parsons.google.utilities import (
     load_google_application_credentials,
@@ -20,9 +21,11 @@
 
 
 class GoogleCloudStorage(object):
-    """
-    This class requires application credentials in the form of a json. It can be passed
-    in the following ways:
+    """Google Cloud Storage connector utility
+
+    This class requires application credentials in the form of a
+    json or google oauth2 Credentials object. It can be passed in the
+    following ways:
 
     * Set an environmental variable named ``GOOGLE_APPLICATION_CREDENTIALS`` with the
       local path to the credentials json.
@@ -33,22 +36,43 @@ class GoogleCloudStorage(object):
 
     * Pass in a json string using the ``app_creds`` argument.
 
+    * Generate the google credentials object directly, pass in using the
+      ``app_creds`` argument.
+
+    For example, to pass in credentials from a parent shell that is
+    authenticated with gcloud auth:
+    ```
+    from google.auth import default
+
+    app_creds, _ = default()
+
+    gcs = GoogleCloudStorage(app_creds=app_creds)
+    ```
+
 
     `Args:`
-        app_creds: str
+        app_creds: str, dict, or google.oauth2.credentials.Credentials object
             A credentials json string or a path to a json file. Not required
-            if ``GOOGLE_APPLICATION_CREDENTIALS`` env variable set.
+            if ``GOOGLE_APPLICATION_CREDENTIALS`` env variable set. Can also
+            pass a google oauth2 Credentials object directly.
         project: str
             The project which the client is acting on behalf of. If not passed
             then will use the default inferred environment.
     `Returns:`
         GoogleCloudStorage Class
+
     """
 
-    def __init__(self, app_creds=None, project=None):
-        env_credentials_path = str(uuid.uuid4())
-        setup_google_application_credentials(app_creds, target_env_var_name=env_credentials_path)
-        credentials = load_google_application_credentials(env_credentials_path)
+    def __init__(self, app_creds: Optional[Union[str, dict, Credentials]] = None, project=None):
+        if isinstance(app_creds, Credentials):
+            credentials = app_creds
+        else:
+            env_credentials_path = str(uuid.uuid4())
+            setup_google_application_credentials(
+                app_creds, target_env_var_name=env_credentials_path
+            )
+            credentials = load_google_application_credentials(env_credentials_path)
+
         self.project = project
 
         # Throws an error if you pass project=None, so adding if/else statement.