feat: Add multiprocessing and chunked downloading to transfer manager (#1002)

* multiprocessing intitial commit

* added multiprocessing, fixed tests; tests for multiprocessing chunks pending

* unit tests for multiprocessing

* add system tests, fix off-by-one error detected by system tests

* lint

* exclude subprocess mocks from coverage

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* add deadline to system tests

* ban broken grpcio version

* Clarify docstring for threads param

* fix coverage issues and catch warning

* respond to feedback

* more feedback

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: andrewsg

google/cloud/storage/client.py

@@ -130,6 +130,11 @@ def __init__(
         if project is _marker:
             project = None
 
+        # Save the initial value of client_info and client_options before they
+        # are passed along, for use in __reduce__ defined elsewhere.
+        self._initial_client_info = client_info
+        self._initial_client_options = client_options
+
         kw_args = {"client_info": client_info}
 
         # `api_endpoint` should be only set by the user via `client_options`,