Endpoint policy

This MR changes the old policy-endpoint.lua script to link si-audio-adapter and si-node session items with WpSiEndpoints (Instead of linking WpEndpoints together like before)

Since we might want to run both policy scripts simultaneously, a new check has been added in policy-node.lua to discard session items that need to be linked with endpoints. This is determined by checking if the session item node's media.role property is set, and if there is at least 1 endpoint present. If both conditions are met, the session item will be handled by policy-endpoint.lua and not policy-node.lua.

Endpoints can be defined in the 10-default-policy.lua endpoints array, which is empty by default, so the default policy behavior should be the same as current master.

Finally, the si-audio-endpoint session item has also been adjusted to work even when a target is not set, which can be useful if no device nodes are present.