Remove endpoints API and use virtual nomenclature instead

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-client.lua and 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.

cc @ashoks