clickhouse-keeper: add 4-letter command for giving up leadership

[chhetripradeep]

Use case

I would like to proactively switch leader in a keeper cluster before stopping a leader. This is useful in many situations like:

• Hardware maintenance
• Keeper upgrades

Describe the solution you'd like

Currently we have rqld where a node can request to become leader. I am looking for opposite of this command.

Describe alternatives you've considered

Only option right now is to mark the current leader as unhealthy and let raft detect that the leader is down which in turn triggers leader election.

[chhetripradeep]

I would like to pick this task.