KVRocks (not clustered mode) does not support the FAILOVER command from Redis

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/kvrocks/issues) and found no similar issues.

### Motivation

Redis has a [FAILOVER](https://redis.io/docs/latest/commands/failover/) command which can be used to gracefully switch which node is the master node in a multi-node (non-cluster) setup.

The details are [described in the documentation](https://redis.io/docs/latest/commands/failover/), but the gist is that it will gracefully pauses writes on the master until at least one replica catches up, then make that replica a master and make the old master into a replica of it.

A command like this is necessary for doing per-node maintenance windows in a rolling fashion, keeping high availability with minimum downtime, and most importantly, _without losing acknowledged writes_.

Apache KVRocks advertises that you can operate it in high availability mode with Redis Sentinel, so I was quite surprised to see that this feature doesn't exist. I can't really treat this as a high availability system without this feature, as there is no clean way to take down a node for maintenance without causing potential loss of acknowledged writes.

### Solution

Implement a Redis-compatible `FAILOVER` command.

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!