Support Opus as A2DP vendor codec.
The specification for vendor A2DP codec is our Pipewire-specific one, so
it is compatible only with devices running Pipewire.
aptX-LL sink devices may send back mSBC encoded data corresponding to
microphone input. It appears to be enabled when the bidirectional link
is set in the caps, and the device also supports this.
Implement mSBC decoding in the duplex channel.
Tested to be working on Avantree Aria Pro.
Support the low-latency variant of the aptx codec.
The magic mostly seems to be on the device side, since the stream is the
same as standard aptx, but latency is smaller even if stream/packet
sizes are the same.
Sound output latency is noticeably less than with the standard aptx.
Tested on Sennheiser HD 250 / Avantree Aria Pro.
The codec in principle also supports bidirectional duplex streams,
but that is not implemented here.
Subdirectories buffer, control, debug, monitor, pod, support and utils, others
are still missing. Headers are grouped either per subdirectory (e.g. buffer/
gets added to group spa_buffer) or per-file (e.g. spa_json is a separate
group), whatever seemed like the most sensible approach.
