ITL: Support --list argument to check_apt

doc/10-icinga-template-library.md

@@ -197,6 +197,7 @@ apt_exclude             | **Optional.** Exclude packages matching REGEXP from th
 apt_critical            | **Optional.** If the full package information of any of the upgradable packages match this REGEXP, the plugin will return CRITICAL status. Can be specified multiple times.
 apt_timeout             | **Optional.** Seconds before plugin times out (default: 10).
 apt_only_critical       | **Optional.** Only warn about critical upgrades.
+apt_list                | **Optional.** List packages available for upgrade.
 
 
 ### breeze <a id="plugin-check-command-breeze"></a>

itl/command-plugins.conf

@@ -1958,6 +1958,10 @@ object CheckCommand "apt" {
 			set_if = "$apt_only_critical$"
 			description = "Only warn about critical upgrades."
 		}
+		"--list" = {
+			set_if = "$apt_list$"
+			description = "List packages available for upgrade."
+		}
 	}
 
 	timeout = 5m