Move all backends to dynamic libaries loaded with spa_plugin_loader so
new backends not needs changes in pipewire or pipewire dependency to
external code
Change-Id: I702ce047598d0c318d6dc6ac8248062a5c12f643
Mostly uses the existing infrastructure, but the webrtc canceller has a
fixed blocksize, so we:
1. Use the canceller blocksize if configured
2. Accumulate output data in a ringbuffer
3. Push out the data in the required chunk size
Only implement the null echo cancel implementation for now.
And skeleton webrtc echo cancel implementation
It uses 4 streams arranged as:
input ---+---> source
^
|
sink ---+---> output
The output of the source is the filtered input of the input stream
(linked a master source) based on the data going from sink to
the output (linked to a master sink).
All streams are arranged in the same group so that the echo canceler
does not have to deal with clock drift. For echo cancelers that can
handle clock drift we might want place the source and sink chains
in different groups.
