Add methods activate() that is called before first call to run() when
stream starts and deactivate() that is called after last call to run()
when stream stops. This makes it possible for aec-plugins to reset their
state between streams.
Place the methods on the interface so that we can call them.
Rename create to init because that is what it does.
Add support for listener and events so that we can signal property
changes later.
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
