add pushimage proto to build api proto

Fields deduced from https://chromium.googlesource.com/chromiumos/chromite/+/refs/heads/master/scripts/pushimage.py
and https://chromium.googlesource.com/chromiumos/chromite/+/refs/heads/master/cbuildbot/commands.py#2679

BUG=b:174516238
TEST=./generate.py

Cq-Depend: chromium:2567387
Change-Id: I145f442dd74b8bb2857cfa2b4accb43a297a5db1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2567717
Reviewed-by: Alex Klein <saklein@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
Commit-Queue: Jack Neus <jackneus@google.com>
2 files changed