Session item policy

This MR refactors wireplumber to not use endpoints at all. The following changes have been done:

• The si-standard-link session item can work with any session item that implements the WpSiPortInfo interface.
• The si-node, si-audio-adapter and si-audio-convert session items don't implement WpSiEndpoint anymore
• A new si-audio-endpoint session item has been added (needed in order to keep current si-standard-link unit test to pass as it uses the wp_endpoint_create_link API)
• The old create-endpoint.lua script has been removed
• A new create-item.lua script has been added to create all session items.
• A new policy-node.lua scrupt has been added to link nodes with each-other
• Adds an "id" property in the session item base class, this makes handling session items in Lua scripts easier.