util/syspolicy/setting: make setting.RawItem JSON-marshallable

We add setting.RawValue, a new type that facilitates unmarshalling JSON numbers and arrays
as uint64 and []string (instead of float64 and []any) for policy setting values.
We then use it to make setting.RawItem JSON-marshallable and update the tests.

Updates #12687

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

types/opt/value.go

@@ -36,7 +36,7 @@ func ValueOf[T any](v T) Value[T] {
 }
 
 // String implements [fmt.Stringer].
-func (o *Value[T]) String() string {
+func (o Value[T]) String() string {
 	if !o.set {
 		return fmt.Sprintf("(empty[%T])", o.value)
 	}