Supervisor should send agent_disconnect message on shutdown

[kroepke]

Currently the server needs to rely on the online threshold expiring to show a disconnected collector in its actual state.

If we send the agent_disconnect message (which the library might already do) and handle it server side, we can immediately reflect this in its status.

See https://opentelemetry.io/docs/specs/opamp/#agenttoserveragent_disconnect