Command For Retrieving the Admin URL

[sprankhub]

Feature description

Similar to config:show:default-url, I think a new command config:show:admin-url would be helpful. I couldn't find a straightforward way to get the admin URL differently. It is also not included in the sys:info command.

Problem and motivation

If you work on a new system, you usually don't know the admin URL.

Is this something you're interested in working on

Interested, but I'll probably not get to this any time soon :(

[its-leofisher]

@cmuench Mind if I pick this up? Already read through contribution process ;)