Respect audio properties provided by receiver and transcode input audio to match

[postlund]

Short feature/function description I realize that I currently don't deal with audio properties, e.g. sample rate, channel count, correctly. I basically try to output in the same format as the input file, which won't work as the receiver tells us what it supports and we need to match that. It is done via zeroconf properties and I believe the following are of interest:

Property	Type	Typical value	Description
ch	int	2	Number of channels, e.g. 2 is stero
cn	int	0,1,2	Audio codecs supported: 0=PCM, 1=ALAC, 2=AAC, 3=AAC ELD
sr	int	44100	Sample rate
ss	int	16	Sample size in bits

Currently, only ALAC output is supported (not sure how to format PCM and what to send in ANNOUNCE for it). So maybe give an error if it is not supported by receiver. Would be surprised if not all receivers supports it.

All properties are already present in the service config, so should be pretty easy to do.

What needs to be done?

• Read out properties from service config when creating the RAOP client and apply them
• Raise an error in case no supported audio codec is present
• Convert input audio to supported format in case it doesn't match (there's a convert_frame method in miniadudio for this)
• Update documentation

Is this a breaking change? No

Anything else worth knowing? Relates to #1059

[postlund]

Interesting observation is that the python miniaudio library automatically decodes any input to 44100/2/16 when loading a file, so that make things kinda just work. What I need to do is to store values from zeroconf and use them as input to decode_file (i.e. make sure input file is decoded to the format the receiver expects). Later versions of AirPlay (2?) doesn't seem to include these properties in Zeroconf at all. So I assume it's safe to default to 44100/2/16 in these cases. Looks like owntone is doing that.