Feature: Speaker group volume replicate volume of individual speaker

[Alives]

Since I can no longer control my speaker group volume with Google Assistant commands, it'd be great if I could select a primary speaker to use as the "volume knob" for the speaker group. go-chromecast could be set to monitor volume events from that speaker UUID and replicate them to the speaker group UUID. AFAIK I'm able to use Google Assistant to control the volume of 1 speaker still, so this would provide that functionality again.