feat: implement raft snapshot catchup

src/NATS.Server/Raft/RaftNode.cs

@@ -5,6 +5,7 @@ public sealed class RaftNode
     private int _votesReceived;
     private readonly List<RaftNode> _cluster = [];
     private readonly RaftReplicator _replicator = new();
+    private readonly RaftSnapshotStore _snapshotStore = new();
 
     public string Id { get; }
     public int Term => TermState.CurrentTerm;
@@ -77,6 +78,24 @@ public sealed class RaftNode
         Log.AppendReplicated(entry);
     }
 
+    public async Task<RaftSnapshot> CreateSnapshotAsync(CancellationToken ct)
+    {
+        var snapshot = new RaftSnapshot
+        {
+            LastIncludedIndex = AppliedIndex,
+            LastIncludedTerm = Term,
+        };
+        await _snapshotStore.SaveAsync(snapshot, ct);
+        return snapshot;
+    }
+
+    public Task InstallSnapshotAsync(RaftSnapshot snapshot, CancellationToken ct)
+    {
+        Log.ReplaceWithSnapshot(snapshot);
+        AppliedIndex = snapshot.LastIncludedIndex;
+        return _snapshotStore.SaveAsync(snapshot, ct);
+    }
+
     public void RequestStepDown()
     {
         Role = RaftRole.Follower;