Sync airplay and amplipi volumes

[SteveMicroNova]

This is a copy of #337, intentionally made as to not spam those that are in that thread as I document my findings

In #1063 I've implemented a volume synchronization script that works with Spotify, I aim to port that forward in to airplay as well. I've found that you can read airplay side volume changes by using shairport-sync-metadata-reader on one of two files depending on which version of airplay you're using: /tmp/shairport-sync-metadata for ap1 or /tmp/shairport-sync-ap2-metadata for ap2. This leads to some problems, as multiple streams of ap1 share that same metadata file and if we ever solve for #436 then airplay 2 will have the same difficulty.

I need to find a way to split those metadata files out and ideally place them in the correct .config/amplipi/srcs/ folder along the way, and I also need to find if there's a way to back populate AmpliPi changes back to the airplay host device as well

[SteveMicroNova]

Change of course: the mpris media player object has a Volume property that can be read that is in sync with the airplay volume, this makes the metadata reader thing irrelevant and removes the need of sorting those two metadata streams into their respective vsrcs