In this PR, we update the syspolicy package to utilize syspolicy/rsop under the hood,
and remove syspolicy.CachingHandler, syspolicy.windowsHandler and related code
which is no longer used.
We mark the syspolicy.Handler interface and RegisterHandler/SetHandlerForTest functions
as deprecated, but keep them temporarily until they are no longer used in other repos.
We also update the package to register setting definitions for all existing policy settings
and to register the Registry-based, Windows-specific policy stores when running on Windows.
Finally, we update existing internal and external tests to use the new API and add a few more
tests and benchmarks.
Updates #12687
Signed-off-by: Nick Khyl <nickk@tailscale.com>
Nick Khyl