Sync AirPlay volume with AmpliPi volume

[Lohrer]

Currently AirPlay has independent volume controls, and can only reduce the volume of audio playing on AmpliPi. Better integration should be achieved so that AirPlay directly controls the connected Zone/Group volumes, and is updated when something else changes AmpliPi's volume.

[linknum23]

Looking at that shairport-sync's interface now that we are finished integrating airplay2.

The configuration file has run_this_when_volume_is_set and ignore_volume_control that seem like they would be very useful for this if they worked together.

Here is the volume control portion of their example configuration file:

//	ignore_volume_control = "no"; // set this to "yes" if you want the volume to be at 100% no matter what the source's volume control is set to.
//	volume_range_db = 60 ; // use this advanced setting to set the range, in dB, you want between the maximum volume and the minimum volume. Range is 30 to 150 dB. Leave it commented out to use mixer's native range.
//	volume_max_db = 0.0 ; // use this advanced setting, which must have a decimal point in it, to set the maximum volume, in dB, you wish to use.
//		The setting is for the hardware mixer, if chosen, or the software mixer otherwise. The value must be in the mixer's range (0.0 to -96.2 for the software mixer).
//		Leave it commented out to use mixer's maximum volume.
//	volume_control_profile = "standard" ; // use this advanced setting to specify how the airplay volume is transferred to the mixer volume.
//		"standard" makes the volume change more quickly at lower volumes and slower at higher volumes.
//		"flat" makes the volume change at the same rate at all volumes.
//	volume_control_combined_hardware_priority = "no"; // when extending the volume range by combining the built-in software attenuator with the hardware mixer attenuator, set this to "yes" to reduce volume by using the hardware mixer first, then the built-in software attenuator.
//	run_this_when_volume_is_set = "/full/path/to/application/and/args"; //	Run the specified application whenever the volume control is set or changed.
//		The desired AirPlay volume is appended to the end of the command line – leave a space if you want it treated as an extra argument.
//		AirPlay volume goes from 0 to -30 and -144 means "mute".

This will need testing ...

[iloveicedgreentea]

This feature would be really nice. Airplay right now defaults to a really low volume and it is annoying to set it to max each time.

[SteveMicroNova]

This functionality is supported by shairport-sync and we have a groundwork being added by #1063 to make this a fairly simple porting over once we find out how shairport-sync exposes the volume info to us