kubo/client/rpc: way to signal node stopping

Something like `api.Closing() <-chan struct{}`, to have a way to tell when daemon is shutting down, etc.

Start by tracing the kubo/client/rpc API and the daemon shutdown lifecycle to determine where a stopping signal would belong. Clarify the signal's lifecycle and consumer expectations; done means the issue's requested way to detect daemon shutdown is defined and exposed through the RPC client API.

Written by the indexing model from the issue text.