Feature: RaftNetwork::snapshot() to send a complete snapshot

[drmingdrmer]

Changelog

Feature: RaftNetwork::snapshot() to send a complete snapshot

Add RaftNetwork::snapshot() to send a complete snapshot and move
sending snapshot by chunks out of ReplicationCore.

To enable a fully customizable implementation of snapshot transmission
tailored to the application's needs, this commit relocates the
chunk-by-chunk transmission logic from ReplicationCore to a new
sub mod, crate::network::stream_snapshot.

The stream_snapshot mod provides a default chunk-based snapshot
transmission mechanism, which can be overridden by creating a custom
implementation of the RaftNetwork::snapshot() method. As part of this
commit, RaftNetwork::snapshot() simply delegates to stream_snapshot.
Developers may use stream_snapshot as a reference when implementing
their own snapshot transmission strategy.

Snapshot transmission is internally divided into two distinct phases:

1. Upon request for snapshot transmission, ReplicationCore initiates a
   new task RaftNetwork::snapshot() dedicated to sending a complete
   Snapshot. This task should be able to be terminated gracefully by
   subscribing the cancel future.

2. Once the snapshot has been fully transmitted by
   RaftNetwork::snapshot(), the task signals an event back to
   ReplicationCore. Subsequently, ReplicationCore informs RaftCore
   of the event, allowing it to acknowledge the completion of the
   snapshot transmission.

Other changes:

• ReplicationCore has two RaftNetworks, one for log replication and
  heartbeat, the other for snapshot only.

• ReplicationClosed becomes a public error for notifying the
  application implemented sender that a snapshot replication is
  canceled.

• StreamingError is introduced as a container of errors that may occur
  in application defined snapshot transmission, including local IO
  error, network errors, errors returned by remote peer and ReplicationClosed.

• The SnapshotResponse type is introduced to differentiate it from the
  InstallSnapshotResponse, which is used for chunk-based responses.

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• Part of Refactor snapshot API #606

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This change is