This adds a new package containing generic types to be used for defining preference hierarchies.
These include prefs.Item, prefs.List, prefs.StructList, and prefs.StructMap. Each of these types
represents a configurable preference, holding the preference's state, value, and metadata.
The metadata includes the default value (if it differs from the zero value of the Go type)
and flags indicating whether a preference is managed via syspolicy or is hidden/read-only for
another reason. This information can be marshaled and sent to the GUI, CLI and web clients
as a source of truth regarding preference configuration, management, and visibility/mutability states.
We plan to use these types to define device preferences, such as the updater preferences,
the permission mode to be used on Windows with #tailscale/corp#18342, and certain global options
that are currently exposed as tailscaled flags. We also aim to eventually use these types for
profile-local preferences in ipn.Prefs and and as a replacement for ipn.MaskedPrefs.
The generic preference types are compatible with the tailscale.com/cmd/viewer and
tailscale.com/cmd/cloner utilities.
Updates #12736
Signed-off-by: Nick Khyl <nickk@tailscale.com>
