build_api: add Tot Firmware Builder service

To support building firmware targets on ToT in a more lightweight manner,
add a new build api that will simply call into a firmware_builder.py that
is specified by path.

See also go/cros-tot-fw-builder

BUG=b:169178847
TEST=none

Change-Id: I3790dc83c1e50b51b5cbadeee5c504978fd95693
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2430167
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Jett Rink <jettrink@chromium.org>
2 files changed