policy-endpint: add support for stream audio nodes without media.role

This MR allows users to assign a particular endpoint for streams that don't have the 'media.role' property set.

If a stream audio node does not have the 'media.role' property set, the policy will assign the 'Default' media role name to it. In addition to this, if the 'Default' endpoint can not be found, the policy will link the stream audio node with the lowest priority endpoint. Video nodes will always be handled by policy-node.lua, and they will never be handled by policy-endpoint-client.lua