util/syspolicy/source: add package for reading policy settings from external stores

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>

util/syspolicy/internal/loggerx/logger.go

@@ -0,0 +1,46 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package loggerx provides logging functions to the rest of the syspolicy packages.
+package loggerx
+
+import (
+	"log"
+
+	"tailscale.com/types/lazy"
+	"tailscale.com/types/logger"
+	"tailscale.com/util/syspolicy/internal"
+)
+
+const (
+	errorPrefix   = "syspolicy: "
+	verbosePrefix = "syspolicy: [v2] "
+)
+
+var (
+	lazyErrorf   lazy.SyncValue[logger.Logf]
+	lazyVerbosef lazy.SyncValue[logger.Logf]
+)
+
+// Errorf formats and writes an error message to the log.
+func Errorf(format string, args ...any) {
+	errorf := lazyErrorf.Get(func() logger.Logf {
+		return logger.WithPrefix(log.Printf, errorPrefix)
+	})
+	errorf(format, args...)
+}
+
+// Verbosef formats and writes an optional, verbose message to the log.
+func Verbosef(format string, args ...any) {
+	verbosef := lazyVerbosef.Get(func() logger.Logf {
+		return logger.WithPrefix(log.Printf, verbosePrefix)
+	})
+	verbosef(format, args...)
+}
+
+// SetForTest sets the specified errorf and verbosef functions for the duration
+// of tb and its subtests.
+func SetForTest(tb internal.TB, errorf, verbosef logger.Logf) {
+	lazyErrorf.SetForTest(tb, errorf, nil)
+	lazyVerbosef.SetForTest(tb, verbosef, nil)
+}