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>

cmd/k8s-operator/depaware.txt

@@ -871,6 +871,7 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
   LW    tailscale.com/util/cmpver                                    from tailscale.com/net/dns+
         tailscale.com/util/ctxkey                                    from tailscale.com/client/tailscale/apitype+
      💣 tailscale.com/util/deephash                                  from tailscale.com/util/syspolicy/setting
+        tailscale.com/util/def                                       from tailscale.com/ipn/localapi
    L 💣 tailscale.com/util/dirwalk                                   from tailscale.com/metrics
         tailscale.com/util/dnsname                                   from tailscale.com/appc+
         tailscale.com/util/eventbus                                  from tailscale.com/tsd+