Changes:
  * luci.notifier_template(...) validates its name.
  * time.epoch(...) functions to parse timestamp strings into Unix times.
  * Internal digests.star module to calculate SHA256s.

R=iannucci@chromium.org

Change-Id: I58a3d9e11a2b278a0fd1e74169d6c57e048488f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2000986
Auto-Submit: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>