macOS Signing Scripts: Add the pipeline module and invoker script.

The pipeline module orchestrates all the signing operations, and the
invoker script is the command line interface to it.

Bug: 958163
Change-Id: Ibf95ee438322b43b5c186a67cb2c082e08c39a0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600547
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657803}
4 files changed