Feature: add Node information into AppendEntriesRequest request

[drmingdrmer]

Add another field leader_node: Node to AppendEntriesRequest.
This way the follower or learner can perceive the node information even when the
leader is not in the membership config.

https://github.com/datafuselabs/openraft/blob/732304ba080dd22e778749d35e71914bc35ea048/openraft/src/raft/message/append_entries.rs#L14-L27

Why

Openraft allows a leader to be not in the membership.
When a leader commits a new membership config that does not contain itself,
the leader will continue to run for a while. During this period, the leader is
not in the cluster(not a voter or learner).

When a Follower receives write-log request, it just returns an error indicating
the caller to forward the request to the leader. But the follower can not find
out Node information in its local memberhship config.

Thus we must embed the leader Node into the AppendEntriesRequest so that
when a heartbeat or append entry request is received by the follower, The
follower will know the leader's information.

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[drmingdrmer]

Close it.
Transmitting leader node information is application level requirement, and it should not be in the core of a raft implementation.