wpctl: Add get-volume command and functionality

Hey again! Sorry I had to open another merge request because the previous one (!376 (closed)) was from the master branch of my fork. I've now added the functionality to query volume levels of nodes. From what I've tested so far, it seems to be working great. I've attached outputs for reference below.

Screenshots:

Output of wpctl

Output of wpctl get-volume

Output of wpctl get-volume + {@DEFAULT_AUDIO_SOURCE@,@DEFAULT_AUDIO_SINK@,@DEFAULT_VIDEO_SOURCE@,some_nodes_on_mmy_device}