client status

Hey, i was recently using this library for my etcd cluster nodes,
i think there isn't a method to check the etcd client status weather client is down or up

also can we also add a method to get an active etcd client node out of a etcd node cluster, or get an active client from a given multiple etcd host

The issue names no files, tests, or entry points. Start by inspecting the client connection and multi-host handling in python-etcd3, then clarify the expected status and active-node behavior; done should include defined APIs and tests for reachable, unavailable, and clustered etcd hosts.

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