Add calls to support creating changes, as well as editing and publishing
them in gerrit_client and gerrit_util.

Bug=b:182613582

Change-Id: I0514cf08dce63ab29d99d4485d96fa124006326a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2800811
Auto-Submit: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>