cylc list: consider upgrading to show live info

[oliver-sanders]

Context: #3931

After the UID it might be worth considering upgrading cylc list to display "live" data as well as listing namespaces.

$ cylc list workflow//cycle/*
~user/workflow//1/a
~user/workflow//1/b
~user/workflow//1/c
$ cylc list workflow//*:running
~user/workflow//1/a
~user/workflow//1/b

This would use the pre-existing task-pool filtering as used by commands like cylc hold and cylc trigger so could be useful for previewing the impact of CLI commands, etc.

Pull requests welcome!