Simplify endpoints and refactor session item

This MR refactors wireplumber, simplifying a lot the code:

• Removes the WpEndpointStream API. the WpEndpointLink object links WpEndpoint's instead of WpEndpointStream's now.
• Refactors WpSessionItem to inherit from WpObject. The activate and export APIs are gone, you need to use WP_SESSION_ITEM_FEATURE_ACTIVE and WP_SESSION_ITEM_FEATURE_EXPORTED in wp_object_activate instead.
• Adds a wp_session_item_register and wp_session_item_remove to register and unregister session items in the registry. Object managers can now listen for session items.
• Renames si-simple-node-endpoint to si-node.
• Renames si-monitor-endpoint to si-monitor
• Renames si-adapter to si-audio-adapter because it is only meant to be used with audio, not video
• Renames si-convert to si-audio-convert because it is only meant to be used with audio, not video
• Renames adapter's algorithms.c file to audio-utils.c
• Makes WpImplEndpoint and WpImplEndpointLink public so that they can be used in session item modules.