[backup client] Refactor ws creation

Summary:
I found a nice way to simplify the websocket creation on the backup client, that will work even better with the future diffs. get_log_ws_response functions has been merged inside the new create_log_ws_connection function, that is used by both log upload and download.

Depends on D10984

Test Plan: Run the backup integration tests.

Reviewers: marcin, bartek, kamil

Reviewed By: bartek, kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10985