[Remote Commands] Add "Remote Powerwash" command

Currently managed Chrome OS devices support remote commands issued by the admin
on CPanel.  Add a command that starts a powerwash on the device when received.

Design doc @ go/remote-powerwash-command

Bug: 891222
Change-Id: I8aab58db3de12681fd655642e984cff6f24e392f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729219
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Ivan Šandrk <isandrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686378}