[skylab_test] add more specs for skylab tests variant

Add 'cros_chrome_version' to compare ash milestone with lacros milestone,
Add 'enabled' to be used to disable the variant used in skylab tests.

But both are not used in skylab recipe, so both are removed in the
generate_buildbot_json when generating json files.

The design doc is here, we chose the option 4. https://docs.google.com/document/d/105Q-M1mIIxhRkJDj65kpfbLgxIqKtI_7h4lelJXcQsA/edit?resourcekey=0-ijBIDu2NrzLporVko7d9PA#heading=h.fo1g9ph6y7ls

Bug: 1253614
Change-Id: I5e164f39582d4fcca907a660265baef23d5f0a1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3292248
Commit-Queue: Jieting Yang <yjt@google.com>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Xinan Lin <linxinan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#946667}