feat(playout): load config using shared helpers

- backport >=py38 Literal type
- update configuration sample
- add missing config filepath to systemd service
- make config sections optional

BREAKING CHANGE: the playout config schema has been changed like the following:
- the top section 'pypo' was renamed to 'playout'
- the 'playout.ls_*' entries were renamed to 'playout.liquidsoap_*'
- the 'playout.*_dir' entries were removed
- the 'playout.api_client' entry was removed
- the 'playout.record_file_type' entry was renamed to 'playout.record_file_format'
- the 'playout.base_recorded_files' entry was removed
- the 'playout.poll_interval' entry was removed
- the 'playout.push_interval' entry was removed
- the 'playout.cue_style' entry was removed

playout/install/systemd/libretime-liquidsoap.service

@@ -3,6 +3,7 @@ Description=Libretime Liquidsoap Service
 
 [Service]
 Environment=LIBRETIME_LOG_FILEPATH=/var/log/libretime/liquidsoap.log
+Environment=LIBRETIME_CONFIG_FILEPATH=/etc/airtime/airtime.conf
 
 ExecStart=/usr/local/bin/libretime-liquidsoap
 User=libretime-playout

playout/install/systemd/libretime-playout.service

@@ -4,6 +4,7 @@ After=network-online.target
 
 [Service]
 Environment=LIBRETIME_LOG_FILEPATH=/var/log/libretime/playout.log
+Environment=LIBRETIME_CONFIG_FILEPATH=/etc/airtime/airtime.conf
 WorkingDirectory=/var/lib/libretime/playout
 
 ExecStart=/usr/local/bin/libretime-playout