Introduces Disconnect block into the TaskGroup configuration (#19886)

This PR is the first on two that will implement the new Disconnect block. In this PR the new block is introduced to be backwards compatible with the fields it will replace. For more information refer to this RFC and this ticket.

command/testdata/example-short-bad.json

@@ -90,6 +90,10 @@
                 "ShutdownDelay": null,
                 "StopAfterClientDisconnect": null,
                 "MaxClientDisconnect": null,
+                "Disconnect":{
+                    "StopAfterClient": null,
+                    "LostAfter": null
+                },
                 "Scaling": null,
                 "Consul": null
             }

command/testdata/example-short.json

@@ -93,6 +93,7 @@
                 "ShutdownDelay": null,
                 "StopAfterClientDisconnect": null,
                 "MaxClientDisconnect": null,
+                "Disconnect": null,
                 "Scaling": null,
                 "Consul": null
             }