Build API: Add synched directory mechanism.

Add functionality for a synced directory. The directory will have
its contents copied in and then out of the chroot before and after
the endpoint is called.

BUG=chromium:1031259
TEST=./generate.sh

Change-Id: Idcc411a78f4be06806f27a9e4f8a752351e543b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1955690
Commit-Queue: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
Reviewed-by: Michael Mortensen <mmortensen@google.com>
5 files changed