music-assistant: Tell players to restart on startup

I haven't fully determined why, but when the Music Assistant server
restarts, it marks the _shairport-sync_ players as offline and will not
allow playing to them.  The only way I have found to work around this is
to restart the players after the server restarts.  As that's pretty
cumbersome and annoying, I naturally want to automate it, so I've
created this rudimentary synchronization technique using _ntfy_: each
player listens for notifications on a specific topic, and upon receiving
one, tells _shairport-sync_ to exit.  With the `Restart=` property
configured on the _shairport-sync.service_ unit, _systemd_ will restart
the service, which causes Music Assistant to discover the player again.

music-assistant/restart-sync.service

@@ -0,0 +1,22 @@
+# vim: set ft=systemd :
+
+# In order for this to be effective, `shairport-sync.service` needs a
+# configuration override to enable auto-restart, e.g.:
+#
+# [Service]
+# Restart=always
+# RestartSec=1
+
+[Unit]
+Description=Restart shairport-sync when music-assistant restarts
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/libexec/restart-sync.sh
+Restart=always
+RestartSec=1
+
+[Install]
+WantedBy=shairport-sync.service