Create new variable var_timesync_service

The variable is used to select the desired timesync service
(systemd-timesync vs chrony) in package/service install/enable
rules when using _guard_var templates.

Analogous to var_network_filtering_service introduced in ComplianceAsCode#11818

linux_os/guide/services/ntp/var_timesync_service.var

@@ -0,0 +1,17 @@
+documentation_complete: true
+
+title: 'Time synchronization service'
+
+description: |-
+   Time synchronization service: systemd-timesyncd or chronyd
+
+type: string
+
+operator: equals
+
+interactive: true
+
+options:
+    systemd-timesyncd: systemd-timesyncd
+    chronyd: chronyd
+    default: systemd-timesyncd