Add GomaArtifacts message; use it in InstallPackagesResponse

This is the list of files added to the goma output_dir specified by
InstallPackagesRequest.

BUG=chromium:1013499
TEST=./generate.sh

Change-Id: I4147a2fbac65b6790a29698005d2df51ce2aa0c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2001194
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Michael Mortensen <mmortensen@google.com>
5 files changed