We add package defining interfaces for policy stores, enabling creation of policy sources
and reading settings from them. It includes a Windows-specific PlatformPolicyStore for GP and MDM
policies stored in the Registry, and an in-memory TestStore for testing purposes.
We also include an internal package that tracks and reports policy usage metrics when a policy setting
is read from a store. Initially, it will be used only on Windows and Android, as macOS, iOS, and tvOS
report their own metrics. However, we plan to use it across all platforms eventually.
Updates #12687
Signed-off-by: Nick Khyl <nickk@tailscale.com>
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