Task: Implement leader forwarding functionality.

**Is your feature request related to a problem? Please describe.**
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**Describe the solution you'd like**
```

base-kv/base-kv-raft/src/main/java/org/apache/bifromq/basekv/raft/RaftNodeStateFollower.java

void changeClusterConfig(String correlateId,
Set newVoters,
Set newLearners,
CompletableFuture onDone) {
// TODO: support leader forward
onDone.completeExceptionally(ClusterConfigChangeException.notLeader());
}
```

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**Additional context**
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