util: add parse fallback helpers (#20022)

util/def: add def.Bool and def.Duration default parse helpers

Replace multiple instances of def.Bool and def.Duration with a new util/def
package.

Updates #20018

Co-authored-by: Bobby <boby@codelabs.co.id>
Co-authored-by: Simon Law <sfllaw@tailscale.com>
Signed-off-by: Bobby <boby@codelabs.co.id>
Signed-off-by: Simon Law <sfllaw@tailscale.com>

util/def/def.go

@@ -0,0 +1,34 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package def parses strings with fallback default values.
+package def
+
+import (
+	"strconv"
+	"time"
+)
+
+// Bool parses s as a bool, returning def when s is empty or invalid.
+func Bool(s string, def bool) bool {
+	if s == "" {
+		return def
+	}
+	v, err := strconv.ParseBool(s)
+	if err != nil {
+		return def
+	}
+	return v
+}
+
+// Duration parses s as a time.Duration, returning def when s is empty or invalid.
+func Duration(s string, def time.Duration) time.Duration {
+	if s == "" {
+		return def
+	}
+	v, err := time.ParseDuration(s)
+	if err != nil {
+		return def
+	}
+	return v
+}