This MR removes the WpEndpoint API because it was never really used. The
si-audio-endpoint session item has also been renamed to
si-audio-virtual to avoid confusions.
Finally, the old policy that was handling
si-audio-endpoint session items has been also ported to the event stack. This basically removes both the
policy-endpoint-device.lua scripts, and creates a new
linking/find-virtual-target.lua script to link clients with
si-audio-virtual session items if one of them can be found. The
policy-endpoint-client-links.lua has also been refactored into a new
scripts/rescan-virtual-links.lua to use the event stack. The idea is for the
scripts/link-target.lua to create all links but only activate non virtual links, and for the
scripts/rescan-virtual-links.lua to activate/deactivate all virtual links based on role priorities.