Add GSC to the set of image types

The new image type will be used for all Google security chip signing
requests.

golang protobufs were generated by running ./generate.sh.

BUG=b:173049030
TEST=along with the upcoming changes in the chromite directory
     successfully invoked 'trigger_gsc_signing'

Change-Id: I20dfa0c38818fc0910df25af95a0e3a3fac84566
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2566418
Auto-Submit: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
Commit-Queue: George Engelbrecht <engeg@google.com>
4 files changed