Add a "client-id" subcommand to yggctl

[subpop]

While there are other ways to acquire the active client ID of a yggd instance, having a canonical way to fetch it through a command like yggctl client-id makes it easy to embed into commands. For example:

pub -topic yggdrasil/$(yggctl client-id)/data/in

In order for yggctl to acquire the current client ID, yggd probably needs to export a property on the com.redhat.Yggdrasil1 D-Bus interface.