feat: initial support for online/offline status via systemd-networkd

[jokeyrhyme]

• I run COSMIC on Arch Linux without network-manager with wpa_supplicant, instead preferring to use systemd-networkd with iwd
• cosmic-greeter currently shows a network indicator using information retrieved from network-manager (via D-Bus)
• this PR adds an optional dependency on the https://crates.io/crates/zbus_systemd crate which is maintained by some of the zbus folks
• this PR uses this crate to listen for signals from the systemd-networkd D-Bus service and otherwise interrogate it to determine the online/offline status, to feed the same network indicator that would otherwise be populated by the data from network-manager
• I have not tested this on a system that has both network-manager and systemd-networkd in an operational state (how common is this setup?)
• I do intend to follow-up with the missing wireless-signal-strength functionality (and later apply this same work to cosmic-applets)

[jokeyrhyme]

Alternatively, perhaps the network-manager versus systemd-networkd concern should be abstracted away so it doesn't belong here?