This CL adds a non-functional `--protocol` flag to fetch.py. This will be used in the follow-up CL to update fetching protocol.

Bug: 1322156
Change-Id: I7eeebb9993face20bb671d6942ee23fd04802d8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3609453Reviewed-by: Joanna Wang <jojwang@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>