Unprivileged read-only interface for observing active sessions. Sent once when the manager is first created. May be an empty array. Broadcast to all connected clients when any session is created. Broadcast to all connected clients when any session is destroyed. Creates a `session_core_session` object for the given session_id. The daemon immediately pushes all fields, followed by done. If the session does not exist, destroyed is sent instead. Creates a session_core_session object for the session whose leader PID matches the given pid. The daemon immediately pushes all fields, followed by done. If no matching session exists, destroyed is sent instead. Creates a session_core_session object for the session whose user ID matches the given uid. The daemon immediately pushes all fields, followed by done. If no matching session exists, destroyed is sent instead. Provides information about a single session. All fields are pushed immediately on creation and re-sent if they change. Sent when the session is destroyed or if the `session_id` passed to `get_session` was not found. The client should release this object.