util/syspolicy/setting: add package that contains types for the next syspolicy PRs

Package setting contains types for defining and representing policy settings.
It facilitates the registration of setting definitions using Register and RegisterDefinition,
and the retrieval of registered setting definitions via Definitions and DefinitionOf.
This package is intended for use primarily within the syspolicy package hierarchy,
and added in a preparation for the next PRs.

Updates #12687

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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